feat(f6): diff-view toggle on any file — global storage, untitled in-memory (#19)

F6 no longer requires a workspace file. It gets its own diffable predicate
(any file: or untitled:) decoupled from F2 isTracked, keys baselines by a
sha256 of the document URI, and stores them in VS Code GLOBAL storage
(context.globalStorageUri) — always present, never the repo (INV-19). Untitled
buffers have no durable identity → in-memory baseline only (lost on reload).

DiffViewController is now constructed before the workspace-root check and its
commands are always live (never stubbed) — the machine-landing advance wiring
stays in the with-root branch since the seam only fires on workspace files.
BaselineStore generalized to key-by-hash (Baseline.docPath → uri).

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

src/baselineStore.ts

@@ -1,10 +1,12 @@
 /**
  * BaselineStore — load/save one diff-view baseline JSON per document (F6 §6.3).
  * The CoauthorStore shape (src/store.ts): vscode-free (Node fs only, unit-
- * testable), one file per docPath. INV-19: the storage dir is VS Code's
- * per-workspace extension storage, NEVER the repo — so the baseline can never
- * be committed, merged, or read by another rung, and the sidecar / cross-rung
- * contract (INV-14..17) are untouched by construction.
+ * testable), one file per storage key. INV-19: the storage dir is VS Code's
+ * per-extension GLOBAL storage, NEVER the repo — so the baseline can never be
+ * committed, merged, or read by another rung. F6 works on *any* file, so the
+ * key is a hash of the document URI (the controller derives it), not a
+ * workspace-relative path; an untitled buffer has no durable identity and is
+ * never persisted here (in-memory only — see DiffViewController).
  */
 import * as fs from "node:fs";
 import * as path from "node:path";
@@ -12,7 +14,8 @@ import * as path from "node:path";
 export type BaselineReason = "opened" | "machine-landing" | "pinned";
 
 export interface Baseline {
-  docPath: string;
+  /** the document URI (the identity the key is derived from) — for round-trip/debug. */
+  uri: string;
   /** full document text captured at the epoch (from the buffer, not disk — §6.3). */
   text: string;
   capturedAt: string;
@@ -20,23 +23,23 @@ export interface Baseline {
 }
 
 export class BaselineStore {
-  /** @param storageDir absolute VS Code workspace-storage dir (context.storageUri.fsPath). */
+  /** @param storageDir absolute VS Code GLOBAL-storage dir (context.globalStorageUri.fsPath). */
   constructor(private readonly storageDir: string) {}
 
-  /** `<storageDir>/baselines/<repo-relative-docPath>.json` (§6.3). */
-  baselinePath(docPath: string): string {
-    return path.join(this.storageDir, "baselines", `${docPath}.json`);
+  /** `<storageDir>/baselines/<key>.json` — `key` is a filesystem-safe hash (§6.3). */
+  baselinePath(key: string): string {
+    return path.join(this.storageDir, "baselines", `${key}.json`);
   }
 
-  load(docPath: string): Baseline | null {
-    const p = this.baselinePath(docPath);
+  load(key: string): Baseline | null {
+    const p = this.baselinePath(key);
     if (!fs.existsSync(p)) return null;
     return JSON.parse(fs.readFileSync(p, "utf8")) as Baseline;
   }
 
   /** Newest epoch wins — overwrite in place, no history (state-not-history, the F3/F4 precedent). */
-  save(docPath: string, baseline: Baseline): void {
-    const p = this.baselinePath(docPath);
+  save(key: string, baseline: Baseline): void {
+    const p = this.baselinePath(key);
     fs.mkdirSync(path.dirname(p), { recursive: true });
     fs.writeFileSync(p, JSON.stringify(baseline, null, 2) + "\n", "utf8");
   }