fix(session-persistence): async readRaw default and full branch coverage

The inherited readRaw default now rejects like the async backend overrides
instead of throwing synchronously, and its arms plus the JSONL override's
retry loop, zero-frame, and corrupt-header branches get dedicated tests.

packages/session/session-persistence-jsonl/tests/jsonl.spec.ts

@@ -237,6 +237,26 @@ describe('SessionPersistenceJsonl: durability and crash semantics', () => {
     expect(await ctx.sessionPersistence.readRaw(m.id)).toBeUndefined()
   })
 
+  it('readRaw rejects a corrupt header line instead of exporting it', async () => {
+    const m = meta('raw-corrupt', '/work')
+    await ctx.sessionPersistence.create(m)
+    await ctx.sessionPersistence.append(m.id, oneTurnLog())
+    await writeFile(rawLogPath(root, '/work', m.id), 'not a header line\n{"type":"turn/start","seq":0}\n')
+    await expect(ctx.sessionPersistence.readRaw(m.id)).rejects.toThrow(/corrupt session log/)
+  })
+
+  it('readRaw retries when the file revision changes during the read', async () => {
+    const m = meta('raw-revision-race', '/work')
+    await ctx.sessionPersistence.create(m)
+    await ctx.sessionPersistence.append(m.id, oneTurnLog())
+    statRace.path = rawLogPath(root, '/work', m.id)
+
+    const raw = await ctx.sessionPersistence.readRaw(m.id)
+    expect(raw).toBeDefined()
+    // Two stat calls per iteration; the mocked revision change forces a retry.
+    expect(statRace.reads).toBe(4)
+  })
+
   it('keeps the same location on resume and gives a fork its own location', async () => {
     const parent = meta('location-parent', '/work')
     const parentLocation = ctx.sessionPersistence.locate(parent)