v0.4.0

Breaking: SCHEMA_VERSION 1 -> 2 (FeedStatus gains `stale`).

- Opt-in `stale-if-error` cache policy: a refused revalidation serves the cached
  body as `status: "stale"` with a SERVED_STALE warning instead of failing the
  feed. `--stale-if-error` (CLI) / `cache_policy: "stale-if-error"` (MCP). ADR-0019.
- The batch deadline is now enforced at the per-host gate, so a throttled
  same-host batch returns a bounded partial page instead of outliving the
  caller's tool timeout and returning nothing.
- Enumerated MCP tool arguments carry `enum`/`pattern` schema constraints.
- 48 crates updated; clears RUSTSEC-2026-0258 (h2) and a yanked chacha20.