F3 hardening: observable seam misses, cheaper disk-compare, E2E ordering notes (#6)

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

src/pendingEdits.ts

@@ -55,11 +55,15 @@ export class PendingEditRegistry {
 
   /**
    * Remove a registration that failed to apply. Identity-based (`===`), so
-   * callers must keep the exact registered object; it is a no-op when `match`
-   * already consumed the registration.
+   * callers must keep the exact registered object. Returns `true` if the
+   * registration was still pending and was removed, `false` if it was already
+   * consumed by `match` (or was never registered). This lets callers detect
+   * seam misses for diagnosability (INV-9).
    */
-  unregister(edit: PendingEdit): void {
+  unregister(edit: PendingEdit): boolean {
+    const before = this.pending.length;
     this.pending = this.pending.filter((p) => p !== edit);
+    return this.pending.length < before;
   }
 
   /**