Replace RemoveAt(0)-in-a-loop drain idiom with RemoveRange(0, i) in both
Step-1 deferred drain and the post-found deferred drain inside DrainAndApply,
making each an O(N) single shift instead of O(N²) on the render-thread hot path.
Add PriorityNeverArrives_noThrow_noLoss_noDoubleApply test: sets a priority id
that never appears in the outbox, ticks several times, asserts no throw, no
loss of the non-priority completions, and no double-apply (applied count ==
completions enqueued). Comment above the priority-hunt explains the failure
mode: completions relocate to _deferredApply while the hunt is active and drain
at per-frame budget until the caller clears PriorityLandblockId.
Restyle test 2 to use named constructor arguments and break the compressed
lambda onto readable lines (matches test 1 style).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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>
