feat(zddc): Phase 4a — drop_target cascade key, browse upload zone migrated

The last hardcoded client-side knowledge of the canonical convention
was the upload-zone regex in browse:

    var UPLOAD_SCOPES = /\/(working|staging|incoming)(\/|$)/i;

Now declared in the cascade:

  Schema:
    drop_target: true|false   leaf-only; describes THIS dir
                              (not propagated to descendants)

  Lookup:
    zddc.DropTargetAt(root, dir) bool

  Surfaced to clients:
    Directory listings carry an X-ZDDC-Drop-Target: true response
    header when the cascade declares this leaf as an upload zone.
    No header = no drop target.

  Defaults populated:
    working / working/* / staging / archive/<party>/incoming
    all carry drop_target: true. Operators can extend (e.g. drop
    files on archive/<party>/received via override) or disable
    (e.g. drop_target: false at a specific staging subtree) without
    touching code.

  Browse migration:
    loader.fetchServerChildren reads the response header and stamps
    state.scopeDropTarget on every listing fetch. upload.js's
    currentScopeAllows now reads that flag instead of regex-
    matching the URL. Initial value is false in init.js so a
    listing failure (offline / server doesn't emit the header)
    safely defaults to "no drop zone".

Phase 4a closes the most visible asymmetry between server-side and
client-side cascade knowledge. The remaining client hardcodes
(browse grid-mode regex, archive source heuristics, shared/nav
stage strip) follow the same pattern when needed — Phase 4b/c/d.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

browse/js/init.js

@@ -50,6 +50,11 @@
 
         // Single shared popup window for file preview (across
         // multiple file clicks). Same pattern as archive's preview.
-        previewWindow: null
+        previewWindow: null,
+
+        // Cascade-resolved scope flags, refreshed on each listing
+        // fetch from response headers.
+        //   scopeDropTarget: cascade's drop_target at currentPath
+        scopeDropTarget: false
     };
 })();

browse/js/loader.js

@@ -85,6 +85,14 @@
             headers: { 'Accept': 'application/json' },
             credentials: 'same-origin'
         });
+        // Capture cascade-resolved scope flags from response headers
+        // before bailing on 404. zddc-server emits X-ZDDC-Drop-Target
+        // for directories the cascade marks as upload destinations
+        // (see zddc/internal/zddc/lookups.go DropTargetAt). The flag
+        // is leaf-only — it describes THIS path, not its descendants
+        // — so a rescope or popstate re-reads it from the new listing.
+        var dropTargetHdr = (resp.headers.get('X-ZDDC-Drop-Target') || '').toLowerCase();
+        window.app.state.scopeDropTarget = dropTargetHdr === 'true';
         if (resp.status === 404) {
             return [];
         }

browse/js/upload.js

@@ -1,10 +1,16 @@
 // upload.js — drag-drop file upload into the current scope.
 //
-// Active only in server mode and only at paths where uploads make
-// sense (any segment named working / staging / incoming, case-
-// insensitive). At other scopes the handlers stay armed but ignore
-// drops silently — there is no visible drop-zone overlay outside an
-// upload-eligible context.
+// Active only in server mode and only at paths where the cascade
+// declares drop_target: true (see zddc/internal/zddc/lookups.go
+// DropTargetAt + defaults.zddc.yaml). The loader captures the
+// X-ZDDC-Drop-Target response header on every directory listing
+// fetch and stamps state.scopeDropTarget; this module just reads it.
+//
+// At scopes where drop_target is false (or unset), the handlers
+// stay armed but ignore drops silently — no visible drop-zone
+// overlay. An operator can flip working/staging/incoming on or
+// extend the cascade to mark additional dirs as drop targets via
+// .zddc; the client follows automatically without code change.
 //
 // Wire model:
 //   - dragenter on the document raises a counter; first-enter shows
@@ -30,11 +36,6 @@
     if (!window.app || !window.app.modules) return;
 
     var UPLOAD_MAX_BYTES = 256 * 1024 * 1024;       // 256 MiB per file
-    // Path segments where uploads are allowed. Matches the current
-    // hardcoded surface (working / staging / incoming). Will become
-    // configurable when the folders: schema lands.
-    var UPLOAD_SCOPES = /\/(working|staging|incoming)(\/|$)/i;
-
     var state = window.app.state;
     var enterCount = 0;
     var overlayEl = null;
@@ -56,8 +57,12 @@
 
     function currentScopeAllows() {
         if (!state || state.source !== 'server') return false;
-        var p = state.currentPath || '';
-        return UPLOAD_SCOPES.test(p);
+        // state.scopeDropTarget is set by the loader on every listing
+        // fetch from the X-ZDDC-Drop-Target response header; it's a
+        // boolean read of the cascade's effective drop_target flag at
+        // the current path. Defaults to false when the header is
+        // absent (older server or non-server response).
+        return !!state.scopeDropTarget;
     }
 
     function showOverlay() {

zddc/internal/handler/directory.go

@@ -128,6 +128,15 @@ func ServeDirectory(cfg config.Config, appsSrv *apps.Server, w http.ResponseWrit
 	// auto-detect (which fetches the same URL with Accept: JSON).
 	w.Header().Set("Vary", "Accept")
 
+	// Surface cascade-resolved scope flags via response headers so
+	// the browse SPA can render scope-aware UI (drop-zone overlay,
+	// future affordances) without re-implementing the cascade
+	// client-side. Keep the header surface tight — only routing-
+	// shape booleans go here; ACL details stay server-side.
+	if zddc.DropTargetAt(cfg.Root, absDir) {
+		w.Header().Set("X-ZDDC-Drop-Target", "true")
+	}
+
 	if strings.Contains(accept, "application/json") {
 		// Content-hash ETag on the listing payload. Re-fetched on every
 		// request (the cascade is walked, ACL filter applied, JSON

zddc/internal/handler/tables.html

@@ -1300,7 +1300,7 @@ body.help-open .app-header {
             </svg>
             <div class="header-title-group">
                 <span class="app-header__title" id="table-title">ZDDC Table</span>
-                <span class="build-timestamp"><span style="color:red;font-weight:bold">v0.0.17-alpha · 2026-05-11 21:01:13 · 5e393cb-dirty</span></span>
+                <span class="build-timestamp"><span style="color:red;font-weight:bold">v0.0.17-alpha · 2026-05-11 21:11:16 · 6310afa-dirty</span></span>
             </div>
         </div>
         <div class="header-right">

zddc/internal/zddc/defaults.zddc.yaml

@@ -68,6 +68,8 @@ paths:
                 # First write into incoming/ auto-creates an owner
                 # grant so the creator can manage their own drops.
                 auto_own: true
+                # Browse shows a drag-drop overlay here.
+                drop_target: true
               received:
                 default_tool: archive
                 # received/ is WORM — express as ACL elsewhere; the
@@ -80,6 +82,7 @@ paths:
         # working/ auto-owns the first creator + the per-user homes
         # below.
         auto_own: true
+        drop_target: true
         paths:
           "*":           # per-user home dir
             default_tool: mdedit
@@ -90,10 +93,12 @@ paths:
             # grants don't reach inside. The user can edit the file
             # to grant collaborators access.
             auto_own_fenced: true
+            drop_target: true
       staging:
         default_tool: transmittal
         available_tools: [transmittal, classifier]
         auto_own: true
+        drop_target: true
       reviewing:
         default_tool: mdedit
         available_tools: [mdedit]

zddc/internal/zddc/file.go

@@ -201,6 +201,14 @@ type ZddcFile struct {
 	// cascade.
 	Virtual *bool `yaml:"virtual,omitempty" json:"virtual,omitempty"`
 
+	// DropTarget marks this directory as a destination for drag-drop
+	// uploads in the browse client. The directory listing's response
+	// header (X-ZDDC-Drop-Target) surfaces this to the SPA, which
+	// shows the drop-zone overlay only at scopes where the cascade
+	// permits uploads. Leaf-only — the property describes THIS dir,
+	// not its descendants. Defaults (nil): no drop zone.
+	DropTarget *bool `yaml:"drop_target,omitempty" json:"drop_target,omitempty"`
+
 	// AvailableTools restricts which tools the server will auto-serve
 	// at this directory and its descendants. The effective list is the
 	// concat-dedupe union of all AvailableTools across the cascade

zddc/internal/zddc/lookups.go

@@ -68,6 +68,24 @@ func AutoOwnFencedAt(fsRoot, dirPath string) bool {
 	return false
 }
 
+// DropTargetAt reports whether THIS specific directory accepts
+// drag-drop uploads from a client (e.g. browse's drop-zone overlay).
+// Leaf-only — the property describes a specific path, not a subtree.
+func DropTargetAt(fsRoot, dirPath string) bool {
+	chain, err := EffectivePolicy(fsRoot, dirPath)
+	if err != nil {
+		return false
+	}
+	leaf := leafLevel(chain)
+	if leaf.DropTarget != nil {
+		return *leaf.DropTarget
+	}
+	if v := chain.Embedded.DropTarget; v != nil {
+		return *v
+	}
+	return false
+}
+
 // VirtualAt reports whether THIS specific directory is declared as
 // purely virtual (never materialise on disk). Leaf-only: the virtual
 // property describes a particular path, not a subtree. A child of a
@@ -201,7 +219,8 @@ func isZeroZddcFile(zf ZddcFile) bool {
 	if zf.DefaultTool != "" {
 		return false
 	}
-	if zf.AutoOwn != nil || zf.AutoOwnFenced != nil || zf.Virtual != nil || zf.Inherit != nil {
+	if zf.AutoOwn != nil || zf.AutoOwnFenced != nil || zf.Virtual != nil ||
+		zf.DropTarget != nil || zf.Inherit != nil {
 		return false
 	}
 	if len(zf.AvailableTools) > 0 {

zddc/internal/zddc/walker.go

@@ -75,6 +75,9 @@ func mergeOverlay(base, top ZddcFile) ZddcFile {
 	if top.AutoOwnFenced != nil {
 		out.AutoOwnFenced = top.AutoOwnFenced
 	}
+	if top.DropTarget != nil {
+		out.DropTarget = top.DropTarget
+	}
 	if top.Virtual != nil {
 		out.Virtual = top.Virtual
 	}