LEASE_CAPTURE_ONLY: group leases of the corpus windows' ranges only

The lease bounds alone don't select a window: the history planner's oldest
buckets are priority 0-2 as well, so the window worker at 0..0 also took 2026
history at 0 (25k ranges in 40 groups, fetched live) while 1,739 ranges of the
2016 corpus window sat at voyager 2's leftover priorities (done when the window
was marked, planned again by the reset). With LEASE_CAPTURE_ONLY the lease, next
and the prefetcher's run-ahead take capture ranges only; the run-ahead now also
keeps to the lease bounds. Test: a group with history at 0 only isn't leased,
the history range after the window isn't handed out.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

start/env.ts

@@ -77,6 +77,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   SNAPSHOT_DIR: Env.schema.string.optional(),
   LEASE_MIN_PRIORITY: Env.schema.number.optional(),
   LEASE_MAX_PRIORITY: Env.schema.number.optional(),
+  LEASE_CAPTURE_ONLY: Env.schema.boolean.optional(),
   PENDING_SHARE: Env.schema.number.optional(),
   PENDING_MIN_PROBES: Env.schema.number.optional(),
   PENDING_MAX_NAMED: Env.schema.number.optional(),