fix(session-persistence): scope the ownerless-state claim to the cwd (review)

A reviewer found a cross-cwd hole: the ownerless-state claim path validated only
the seed prefix (via loadStored, any scope) and never compared the tracked
header's cwd to the live session's. So an ownerless `create(meta(id, "/a"))`
with cursor 0 (seed matches trivially) was claimed by a live session with the
same id at cwd "/b", and the "/b" events then appended under the "/a" header —
bypassing the cwd-scoped loadLive() guard that the HMR-adopt path (case 2) uses.

Add a cwd equality check before the seed check in the ownerless-claim branch: a
same-id ownerless artifact at a different cwd is a collision, not a claim. This
is a coordinator-level invariant (the live session's cwd must match the tracked
meta's cwd) and applies to both backends.

Tests (shared coordinator contract, run per backend): a live session at a
different cwd cannot claim cursor-0 ownerless state, cannot claim a
loaded-prefix even when the seed matches, and a no-cwd state cannot be claimed
by a cwd'd session. All fail without the guard.

Also documents WHY the `materialized` flag is needed (lazy create leaves no
artifact; it distinguishes registered-but-unwritten from durably-present for
has()/reclaim) and reframes the module doc to current-state, not the refactor
history (per the new AGENTS.md doc convention).

packages/session-persistence/tests/coordinator-contract.ts

@@ -68,6 +68,7 @@ export interface CoordinatorFixture {
 
 /** A constant absolute cwd; jsonl keys directories off it, memory/sqlite ignore it. */
 const WORK = '/w'
+const OTHER = '/other'
 
 /** The per-session init map a backend exposes for white-box init awaits. */
 function inits(persistence: SessionPersistence): Map<Session, Promise<void>> {
@@ -535,6 +536,58 @@ export function runCoordinatorContract(name: string, makeFixture: () => Promise<
       }
     })
 
+    it('a live session at a DIFFERENT cwd cannot claim cursor-0 ownerless state (cwd scope)', async () => {
+      const fix = await makeFixture()
+      const { ctx, fiber } = await freshCtx(fix)
+      try {
+        // create() registers ownerless state at cwd /a (cursor 0 — claims would
+        // otherwise match trivially on the seed).
+        await ctx.sessionPersistence.create(meta('wrong-cwd-claim', OTHER))
+        // A live session reusing the id but at cwd WORK must NOT claim it — the
+        // cwd scope is the fence (without it, WORK events would append under the
+        // OTHER header). Rejected as a collision.
+        const live = ctx.sessions.create('wrong-cwd-claim', { seed: oneTurnLog(), meta: { cwd: WORK } })
+        await expect(inits(ctx.sessionPersistence).get(live)).rejects.toThrow(/different cwd|id collision/)
+      } finally {
+        await fiber.dispose()
+        await fix.cleanup()
+      }
+    })
+
+    it('a live session at a DIFFERENT cwd cannot claim loaded-prefix ownerless state (cwd scope)', async () => {
+      const fix = await makeFixture()
+      const { ctx, fiber } = await freshCtx(fix)
+      try {
+        // Materialize + load at cwd OTHER (ownerless, cursor = 6).
+        await ctx.sessionPersistence.create(meta('wrong-cwd-load', OTHER))
+        await ctx.sessionPersistence.append(SessionId('wrong-cwd-load'), oneTurnLog())
+        const { events } = await ctx.sessionPersistence.load(SessionId('wrong-cwd-load'))
+        // A live session whose SEED matches the loaded prefix but whose cwd is
+        // WORK must still be rejected — the cwd guard runs before the seed check.
+        const live = ctx.sessions.create('wrong-cwd-load', { seed: events, meta: { cwd: WORK } })
+        await expect(inits(ctx.sessionPersistence).get(live)).rejects.toThrow(/different cwd|id collision/)
+      } finally {
+        await fiber.dispose()
+        await fix.cleanup()
+      }
+    })
+
+    it('a no-cwd ownerless state cannot be claimed by a live session WITH a cwd (cwd scope, undefined side)', async () => {
+      const fix = await makeFixture()
+      const { ctx, fiber } = await freshCtx(fix)
+      try {
+        // Ownerless state created WITHOUT a cwd (the no-cwd bucket).
+        await ctx.sessionPersistence.create(meta('no-cwd-state'))
+        // A live session reusing the id but WITH cwd WORK is a cwd mismatch
+        // (undefined vs WORK) and must be rejected.
+        const live = ctx.sessions.create('no-cwd-state', { seed: oneTurnLog(), meta: { cwd: WORK } })
+        await expect(inits(ctx.sessionPersistence).get(live)).rejects.toThrow(/different cwd|id collision/)
+      } finally {
+        await fiber.dispose()
+        await fix.cleanup()
+      }
+    })
+
     // --- append adopts a storage-only session (fresh instance, no prior create/load) ---
 
     it('append adopts a storage-only session (fresh instance) and continues the seq', async () => {