Idle connections fetch capture ranges into the corpus

The spool only fetches what workers lease soon, in priority order, and the
tail never ends: after a reset the 1,400 capture ranges waited behind it
while 34 of 40 connections idled. Up to CORPUS_AHEAD (8) planned capture
ranges whose chunks aren't all stored are now targets after everything else,
fetched into the corpus only (no spool); a later lease reads them from disk.

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(),
+  CORPUS_AHEAD: Env.schema.number.optional(),
   PROBE_PIPELINE: Env.schema.number.optional(),
   SPOOL_AHEAD_PER_LEASE: Env.schema.number.optional(),
   SPOOL_MIN_AHEAD: Env.schema.number.optional(),