fix: address PR #1802 review round

- listChildren reads the session store via strict ctx.get (property proxy
  is caller-scoped), orders candidates branchlessly, narrows the cold-read
  return type, and pins the cost model and store/registry composition gaps
  with tests; per-file coverage restored
- acp-agent and headless-agent compositions mount session-projection; a
  keyless snapshot pins the descriptor-less diagnostic row
- api-proxy cold spec pins header-origin ownership and the legacy
  descriptor-only opt-out
- design note ships as implemented with its English pairing; companion
  notes and core-data-structures pages synced

examples/package.json

@@ -54,6 +54,7 @@
     "@deepseek-ai/dsh-session": "workspace:*",
     "@deepseek-ai/dsh-session-checkpoint-policy": "workspace:*",
     "@deepseek-ai/dsh-session-persistence-jsonl": "workspace:*",
+    "@deepseek-ai/dsh-session-projection": "workspace:*",
     "@deepseek-ai/dsh-session-query": "workspace:*",
     "@deepseek-ai/dsh-session-query-sqlite": "workspace:*",
     "@deepseek-ai/dsh-session-reference": "workspace:*",