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/extension.ts

@@ -10,6 +10,7 @@ import { BaselineStore } from "./baselineStore";
 import { GlobalSidecarStore } from "./globalSidecarStore";
 import { SidecarRouter } from "./sidecarRouter";
 import { DiffViewController } from "./diffViewController";
+import { GitBaselineAdapter } from "./gitBaseline";
 import { TrackChangesPreviewController } from "./trackChangesPreview";
 import { LiveProgressUi } from "./liveProgressUi";
 import { EditorProposalController } from "./editorProposalController";
@@ -87,17 +88,21 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
   );
   coeditingRegistry.syncContext(vscode.window.activeTextEditor);
 
-  // --- F6: baseline data layer (Features #17 + #19) — workspace-INDEPENDENT ---
-  // The two-pane vscode.diff VIEW was removed in #34 (F10's rendered preview is
-  // the single review surface); only the baseline store survives, consumed by
-  // F7/F10. It tracks a baseline for ANY diffable doc (file or untitled), so it
-  // is constructed regardless of whether a folder is open. Baseline lives in VS
-  // Code's per-extension GLOBAL storage (always present), never the repo
-  // (INV-19). The machine-landing advance is wired below. The controller
-  // self-wires baseline capture on open.
+  // --- Baseline router (native-surfaces migration, spec §6.4, INV-7) — workspace-
+  // INDEPENDENT --- The two-pane vscode.diff VIEW was removed in #34 (F10's
+  // rendered preview is the single review surface); only the baseline data
+  // layer survives, consumed by F7/F10. It tracks a baseline for ANY diffable
+  // doc (file or untitled), so it is constructed regardless of whether a folder
+  // is open. A SNAPSHOT baseline lives in VS Code's per-extension GLOBAL
+  // storage (always present), never the repo (INV-19); a git-tracked doc's HEAD
+  // baseline is never persisted (INV-7). Baselines are established only for
+  // documents the CoeditingRegistry has entered — see the registry wiring
+  // inside DiffViewController's constructor.
+  const gitBaseline = new GitBaselineAdapter();
+  context.subscriptions.push(gitBaseline);
   const baselineStorageDir = context.globalStorageUri?.fsPath;
   const baselineStore = baselineStorageDir ? new BaselineStore(baselineStorageDir) : null;
-  const diffViewController = new DiffViewController(baselineStore);
+  const diffViewController = new DiffViewController(baselineStore, gitBaseline, coeditingRegistry);
   context.subscriptions.push(diffViewController);
 
   // F8: the out-of-workspace/untitled authoring sidecar — same GLOBAL storage
@@ -176,12 +181,12 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
     }),
   );
 
-  // --- F6 machine-landing wiring — now for ANY authorable doc ---
-  // The seam's single machine-landing signal advances the F6 baseline (INV-18);
-  // the seam can now fire on out-of-folder files too, so wire it unconditionally.
-  context.subscriptions.push(
-    attributionController.onDidApplyAgentEdit((e) => diffViewController.advance(e.document)),
-  );
+  // NOTE (native-surfaces migration, INV-7/INV-18 retirement): the shipped
+  // machine-landing baseline advance (#48) — wiring
+  // attributionController.onDidApplyAgentEdit → diffViewController.advance —
+  // was removed here. A landed Claude edit now stays a visible change-since-
+  // baseline until the file is committed (head mode) or reviewed via
+  // "Mark Changes as Reviewed" (snapshot mode).
 
   // One SHARED sidecar watcher for both controllers; self-writes are suppressed
   // centrally in the repo store (only repo `.threads/` sidecars are watched —