Prober: pipelined bodies, merge-first order, a probe cache across resets

- XoverClient.bodies: PROBE_PIPELINE (8) BODY commands per connection at
  once, replies read in order (one at a time it ran 3-10 articles/s on 6
  connections against a queue of 1M).
- Claims in tiers: lone files first, then par2 sets and archives of
  releases without a readable name (what the par2 merge needs), then those
  of readable releases.
- probe_cache: final answers by message-id (status, inspection with the
  16 KiB hash and par2 file list), PROBE_VERSION 2. A reset empties the
  probe queue, not the cache; recreated candidates of probed articles are
  answered from it without the news server.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

start/env.ts

@@ -61,6 +61,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   LEASE_TTL_SECONDS: Env.schema.number.optional(),
   SPOOL_DIR: Env.schema.string.optional(),
   CORPUS_DIR: Env.schema.string.optional(),
+  PROBE_PIPELINE: Env.schema.number.optional(),
   SPOOL_AHEAD_PER_LEASE: Env.schema.number.optional(),
   SPOOL_MIN_AHEAD: Env.schema.number.optional(),
   SPOOL_MAX_GB: Env.schema.number.optional(),