docs(editor): add cursor rule for HN-10 heading code guardrails

New .cursor/rules/editor-heading-rules.mdc auto-surfaces INPLACE-1 and
STACK-ATTACH-1 rules when editing changeHeadingLevel*.ts, hierarchy
validation plugin, or heading test files. Links to V-4 history so
future developers understand why delete-then-reinsert is forbidden.

Author: HMarzban

.cursor/rules/editor-heading-rules.mdc

@@ -0,0 +1,33 @@
+---
+description: Heading schema rules (HN-10) — mandatory reading before modifying heading-related editor code
+alwaysApply: false
+globs:
+  [
+    '**/changeHeadingLevel*.ts',
+    '**/hierarchyValidationPlugin.ts',
+    '**/validateHeadingHierarchy.ts',
+    '**/extensions/Heading/**',
+    '**/cypress/e2e/editor/Heading/**'
+  ]
+---
+
+# Editor Heading Rules (HN-10)
+
+Before modifying any heading-related code, you MUST read:
+
+- `Notes/editor/Editor_Schema_Rules_v3.md` — the HN-10 spec (especially **§9 Level-Change Semantics**)
+- `Notes/editor/Code_vs_Rules_Audit.md` — compliance audit and violation history
+
+## Non-Negotiable Rules
+
+1. **INPLACE-1**: Level changes MUST use in-place `setNodeMarkup` — NEVER delete+reinsert. Deleting and reinserting destroys positional identity and causes silent re-parenting under wrong siblings.
+
+2. **STACK-ATTACH-1**: After a forward level change (`newLevel > oldLevel`), re-parent into the preceding sibling's `contentWrapper` if `siblingLevel < newLevel`. This happens in the same transaction, after the in-place attribute change.
+
+3. **Single source of truth**: The hierarchy validation plugin (`appendTransaction`) is the ONLY place that fixes structural violations (child extraction, H1-nested extraction). The level-change command itself MUST NOT duplicate this logic.
+
+4. **H1 special case**: Changing FROM H1 is the only case exempt from INPLACE-1 (section root semantics).
+
+## Why This Matters
+
+A previous implementation used delete-then-reinsert, which caused headings to silently move under wrong siblings — a critical data-corruption bug (V-4). The fix required a full rewrite (Phase 9). See `Code_vs_Rules_Audit.md` for the full history.