feat(f6): onDidApplyAgentEdit — the single machine-landing signal (SLICE-2)

F6 §6.2/§6.4, INV-9/INV-18. Additive EventEmitter on the seam owner, fired
after a real (non-no-op) applyEdit succeeds. Seam signature/return unchanged.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/attributionController.ts

@@ -63,12 +63,21 @@ export class AttributionController implements vscode.Disposable {
   private readonly output = vscode.window.createOutputChannel("Cowriting Attribution");
   private visible = true;
 
+  /**
+   * F6 (§6.2/§6.4): the single machine-landing signal. Fired after a real
+   * (non-no-op) seam apply succeeds. INV-9 makes the seam the sole machine-edit
+   * ingress, so this is the sole signal — DiffViewController subscribes to
+   * advance the baseline; no call-site wiring, no future driver can forget it.
+   */
+  private readonly applyEmitter = new vscode.EventEmitter<{ document: vscode.TextDocument }>();
+  readonly onDidApplyAgentEdit: vscode.Event<{ document: vscode.TextDocument }> = this.applyEmitter.event;
+
   constructor(
     private readonly store: CoauthorStore,
     private readonly rootDir: string,
     private readonly guard: VersionGuard,
   ) {
-    this.disposables.push(this.agentType, this.humanType, this.statusItem, this.output);
+    this.disposables.push(this.agentType, this.humanType, this.statusItem, this.output, this.applyEmitter);
     this.disposables.push(
       vscode.commands.registerCommand("cowriting.toggleAttribution", () => this.toggle()),
       vscode.workspace.onDidChangeTextDocument((e) => this.onDidChange(e)),
@@ -275,6 +284,12 @@ export class AttributionController implements vscode.Disposable {
         "(host minimized differently?) — the edit may be mis-attributed (INV-9).",
       );
     }
+    if (ok) {
+      // F6 (INV-18): a real machine landing — signal the baseline to advance so
+      // this text never shows as a change in the diff view. Fire regardless of
+      // attribution-match bookkeeping above; the landing happened either way.
+      this.applyEmitter.fire({ document });
+    }
     return ok;
   }