Adds PriorityLandblockId (uint, default 0) + _deferredApply buffer.
DrainAndApply now: (1) applies up to budget from the deferred buffer,
(2) when PriorityLandblockId != 0, hunts the worker outbox in chunks
applying the priority LB immediately on match and buffering any
non-priority items drained past it for later frames,
(3) falls back to normal drain when no priority is set or not found.
Extracts ApplyResult(result) + ResultLandblockId(result) helpers so
both the priority and normal paths share identical side-effects.
No existing behaviour changes on the non-priority path.
21 streaming tests pass (19 existing + 2 new priority-apply tests).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
