F3 SLICE-3: pending-edit registry + sidecar section-merge update (INV-9 groundwork) (#6)

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

src/pendingEdits.ts

@@ -0,0 +1,48 @@
+/**
+ * PendingEditRegistry — the seam's bookkeeping (spec §6.2, INV-9). Before
+ * applyAgentEdit applies a WorkspaceEdit, it registers the exact expected
+ * change; the controller's change handler consumes a matching registration to
+ * attribute that change to the agent. Anything unmatched is human typing.
+ * vscode-free and unit-testable.
+ */
+import type { Provenance } from "./model";
+
+export interface PendingEdit {
+  docPath: string;
+  /** half-open [start, end) offsets of the replaced range. */
+  start: number;
+  end: number;
+  newText: string;
+  provenance: Provenance;
+  turnId?: string;
+}
+
+export class PendingEditRegistry {
+  private pending: PendingEdit[] = [];
+
+  register(edit: PendingEdit): void {
+    this.pending.push(edit);
+  }
+
+  /** Remove a registration that failed to apply. */
+  unregister(edit: PendingEdit): void {
+    this.pending = this.pending.filter((p) => p !== edit);
+  }
+
+  /**
+   * Find-and-consume the registration exactly matching a change event
+   * (same doc, same replaced range, same inserted text). Null → human edit.
+   */
+  match(docPath: string, change: { start: number; end: number; text: string }): PendingEdit | null {
+    const i = this.pending.findIndex(
+      (p) =>
+        p.docPath === docPath &&
+        p.start === change.start &&
+        p.end === change.end &&
+        p.newText === change.text,
+    );
+    if (i === -1) return null;
+    const [hit] = this.pending.splice(i, 1);
+    return hit;
+  }
+}