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/zstd.spec.ts

@@ -377,6 +377,18 @@ describe('SessionPersistenceJsonl: default Zstandard encoding', () => {
     expect(scanned.events.map(event => event.type)).toEqual(oneTurnLog().map(event => event.type))
   })
 
+  it('readRaw is undefined for a zstd artifact that carries no frame', async () => {
+    const root = await freshRoot()
+    const ctx = await mount(root)
+    const header = meta('raw-zero-frame', '/work')
+    await ctx.sessionPersistence.create(header)
+    await ctx.sessionPersistence.append(header.id, oneTurnLog())
+    // Overwrite the physical artifact with a short buffer: frame scanning
+    // answers zero frames before any magic check, so readRaw reports no artifact.
+    await writeFile(logPath(root, '/work', header.id, 'zstd'), Buffer.alloc(0))
+    expect(await ctx.sessionPersistence.readRaw(header.id)).toBeUndefined()
+  })
+
   it('resolves the default when a programmatic wrapper bypasses Loader schema normalization', async () => {
     const root = await freshRoot()
     const ctx = new Context()