fix(browse): don't inject front matter on open; no conflict modal on rename

Two fixes to the markdown editor's identity handling:

- Sync-on-open now only RECONCILES front-matter identity keys the author
  already wrote (correcting a stale title/revision/…); it never ADDS them. A
  blank or new file opens blank instead of getting an auto-generated title at
  the top. The converter derives identity from the filename regardless, so an
  empty front matter needs nothing baked in. (Cancel/revert likewise only
  touches existing keys.)

- The "Rename file & reopen" flow force-writes the buffer (no If-Match) instead
  of routing through save(), which raised the conflict-resolution modal on a
  (spurious) 412. The rename is a deliberate user action and the identity edit
  that triggered it is consumed by the new filename, so we commit the buffer
  rather than interrupting with a merge dialog.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

browse/js/preview-markdown.js

@@ -603,16 +603,22 @@
         // even if we tweak whitespace in the YAML lines.
         var initialParsed = parseFrontMatter(text);
         var bodyText = initialParsed.body;
-        // On open, mirror the filename-derived identity into the front matter
+        // On open, RECONCILE existing front-matter identity keys with the
-        // (the filename is the single source of truth; this keeps the values
+        // filename (the single source of truth) — but never ADD them. A blank
-        // baked in for the converter). No-op for non-ZDDC filenames. The dirty
+        // or new file opens blank (we don't inject a title etc.); a file whose
+        // author already wrote a now-stale title/revision/… gets corrected.
+        // The converter derives identity from the filename regardless, so
+        // there's nothing to "bake in" for an empty front matter. The dirty
         // baseline stays the ON-DISK state, so a correction opens the buffer
         // dirty and a save persists it.
         var onDiskFM = stringifyFrontMatter(initialParsed.data);
         var fid = filenameIdentity(node.name);
         if (fid) {
             for (var ik in fid) {
-                if (Object.prototype.hasOwnProperty.call(fid, ik)) initialParsed.data[ik] = fid[ik];
+                if (Object.prototype.hasOwnProperty.call(fid, ik)
+                    && Object.prototype.hasOwnProperty.call(initialParsed.data, ik)) {
+                    initialParsed.data[ik] = fid[ik];
+                }
             }
         }
         var syncedFM = stringifyFrontMatter(initialParsed.data);
@@ -891,7 +897,10 @@
             if (!fid) return;
             var data = parseFrontMatter('---\n' + fmCM.getValue() + '\n---\n').data || {};
             for (var k in fid) {
-                if (Object.prototype.hasOwnProperty.call(fid, k)) data[k] = fid[k];
+                if (Object.prototype.hasOwnProperty.call(fid, k)
+                    && Object.prototype.hasOwnProperty.call(data, k)) {
+                    data[k] = fid[k];
+                }
             }
             fmCM.setValue(stringifyFrontMatter(data));
             var body = editor.getMarkdown();
@@ -909,11 +918,26 @@
         async function renameToMatch(newName) {
             var up = window.app.modules.upload;
             if (!up || !up.renameNode || !newName) return;
-            // 1. Save first so body/FM edits survive the rename. A failed save
+            // 1. Persist the current buffer first so body edits survive the
-            //    (conflict, ACL) leaves the buffer dirty — abort the rename.
+            //    rename. Force the write (no If-Match) — the user deliberately
-            if (instance.dirty) {
+            //    initiated this rename, so we commit their version rather than
-                await save();
+            //    interrupting with the conflict-resolution modal (which save()
-                if (currentInstance !== instance || instance.dirty) return;
+            //    raises on a 412). The identity edit that triggered the rename
+            //    is consumed by the new filename, so there's nothing to merge.
+            if (instance.dirty && canSave(node)) {
+                try {
+                    var content = assembleContent(fmCM.getValue(), editor.getMarkdown());
+                    statusEl.textContent = 'Saving…';
+                    var res = await saveContent(node, content, { force: true });
+                    await markSaved(content, res);
+                    if (currentInstance !== instance) return;
+                } catch (e) {
+                    statusEl.textContent = '';
+                    if (window.zddc && window.zddc.toast) {
+                        window.zddc.toast('Save failed: ' + (e.message || e), 'error');
+                    }
+                    return;
+                }
             }
             // 2. Rename on disk.
             try {