docs: document SessionRawArtifact and fix backpressure claims

The persistence subsystem page gains the SessionRawArtifact section (with a
type-equiv manifest entry and zh counterpart), and the README and Agent Note
describe the drain-based backpressure instead of claiming the archive never
accumulates.

scripts/type-equiv.manifest.json

@@ -440,6 +440,11 @@
       "symbol": "SessionLocation",
       "source": "packages/session/session-persistence/src/index.ts"
     },
+    {
+      "doc": "docs/subsystems/persistence.md",
+      "symbol": "SessionRawArtifact",
+      "source": "packages/session/session-persistence/src/index.ts"
+    },
     {
       "doc": "docs/subsystems/session-query.md",
       "symbol": "SessionEventSurface",