feat(session-persistence): expose readRaw for per-session artifacts

The persistence contract gains a concrete readRaw default (undefined for
backends without a per-session artifact) and the JSONL backend overrides it
with the decode of its physical zstd frames, so a consumer can read the
stored artifact text verbatim — the session-log export depends on it.

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

@@ -356,6 +356,27 @@ describe('SessionPersistenceJsonl: default Zstandard encoding', () => {
     expect((await ctx.sessionPersistence.load(header.id)).events).toEqual(oneTurnLog())
   })
 
+  it('readRaw decodes the compressed artifact back to the original JSONL text', async () => {
+    const root = await freshRoot()
+    const ctx = await mount(root)
+    const header = meta('raw-read-zstd', '/work')
+    await ctx.sessionPersistence.create(header)
+    await ctx.sessionPersistence.append(header.id, oneTurnLog())
+
+    const raw = await ctx.sessionPersistence.readRaw(header.id)
+    expect(raw).toBeDefined()
+    // The logical name drops the physical encoding suffix.
+    expect(raw!.filename).toBe('session.jsonl')
+    expect(raw!.meta.id).toBe(header.id)
+    expect(raw!.content).toBe([
+      JSON.stringify(toHeaderLine(header)),
+      ...oneTurnLog().map(e => JSON.stringify(e)),
+      '',
+    ].join('\n'))
+    const scanned = scanLog(Buffer.from(raw!.content))
+    expect(scanned.events.map(event => event.type)).toEqual(oneTurnLog().map(event => event.type))
+  })
+
   it('resolves the default when a programmatic wrapper bypasses Loader schema normalization', async () => {
     const root = await freshRoot()
     const ctx = new Context()