feat: baseline router — git HEAD + snapshot per INV-7/D13/D14; retire machine-landing advance (spec §6.4)

Task 2 of the native-surfaces migration plan. GitBaselineAdapter resolves a
document's HEAD blob via the built-in vscode.git extension (hardened with a
.git/logs/HEAD watch that nudges repo.status(), since the git extension's own
watcher doesn't reliably notice out-of-band commits on non-workspace-folder
repos in the E2E host). DiffViewController is reworked into a baseline router:
head mode (git-tracked, never persisted, re-read on commit) vs snapshot mode
(captured on CoeditingRegistry entry, re-pinned by "Mark Changes as Reviewed"
— renamed from cowriting.pinDiffBaseline, D14). The shipped machine-landing
baseline advance (#48/INV-18) is retired: a landed Claude edit now stays a
visible change-since-baseline until commit or review (INV-7/D21). Legacy
on-disk reasons ("opened"/"machine-landing") migrate to "entered" via
BaselineStore.normalizeReason on load.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/attributionController.ts

@@ -56,8 +56,11 @@ export class AttributionController implements vscode.Disposable {
   /**
    * 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.
+   * ingress, so this is the sole signal. Native-surfaces migration (INV-7/
+   * INV-18 retirement, spec §6.4): DiffViewController no longer subscribes
+   * here to advance the baseline — a landed edit stays a visible
+   * change-since-baseline until commit (head mode) or "Mark Changes as
+   * Reviewed" (snapshot mode). Other consumers may still subscribe.
    */
   private readonly applyEmitter = new vscode.EventEmitter<{ document: vscode.TextDocument }>();
   readonly onDidApplyAgentEdit: vscode.Event<{ document: vscode.TextDocument }> = this.applyEmitter.event;
@@ -291,9 +294,10 @@ export class AttributionController implements vscode.Disposable {
       );
     }
     if (ok && opts?.landBaseline !== false) {
-      // F6 (INV-18): a real machine landing — advance the baseline. F12 (INV-48)
-      // suppresses this for optimistic apply: the proposed text is in the buffer
-      // but the change stays PENDING (baseline at pre-proposal) until accept.
+      // F6 (INV-18, retired as a baseline advance — spec §6.4/INV-7): fire the
+      // machine-landing signal. F12 (INV-48) suppresses this for optimistic
+      // apply: the proposed text is in the buffer but the change stays PENDING
+      // until accept.
       this.applyEmitter.fire({ document });
     }
     return ok;
@@ -302,8 +306,10 @@ export class AttributionController implements vscode.Disposable {
   /**
    * F12/#64 (INV-51): fire the machine-landing signal WITHOUT applying text — used
    * by finalize-in-place, where the proposed text already landed in the buffer via
-   * optimistic apply (`landBaseline:false`) and accept only needs to advance the
-   * F6 baseline so the now-accepted change stops reading as pending.
+   * optimistic apply (`landBaseline:false`) and accept marks the landing so the
+   * now-accepted change is recorded (INV-7: the baseline itself no longer
+   * advances on landing — it advances only on commit / "Mark Changes as
+   * Reviewed").
    */
   signalLanded(document: vscode.TextDocument): void {
     if (this.isTracked(document)) this.applyEmitter.fire({ document });