feat(f6): BaselineStore — vscode-free per-doc baseline persistence (SLICE-1)

F6 §6.2/§6.3, INV-19. One JSON per docPath under VS Code workspace storage,
never the repo. Mirrors CoauthorStore; unit-tested round-trip/paths/overwrite.

Also lands the F6 implementation plan
(docs/superpowers/plans/2026-06-11-f6-diff-view-toggle.md).

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

src/baselineStore.ts

@@ -0,0 +1,43 @@
+/**
+ * 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.
+ */
+import * as fs from "node:fs";
+import * as path from "node:path";
+
+export type BaselineReason = "opened" | "machine-landing" | "pinned";
+
+export interface Baseline {
+  docPath: string;
+  /** full document text captured at the epoch (from the buffer, not disk — §6.3). */
+  text: string;
+  capturedAt: string;
+  reason: BaselineReason;
+}
+
+export class BaselineStore {
+  /** @param storageDir absolute VS Code workspace-storage dir (context.storageUri.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`);
+  }
+
+  load(docPath: string): Baseline | null {
+    const p = this.baselinePath(docPath);
+    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);
+    fs.mkdirSync(path.dirname(p), { recursive: true });
+    fs.writeFileSync(p, JSON.stringify(baseline, null, 2) + "\n", "utf8");
+  }
+}