feat(tasks): #408 increment 2 — mailboxes (task_send/recv/try_recv/kill)

[InauguralPhysicist]

Refs #408. Cooperative tasks can now communicate — unbounded FIFO mailboxes with share-nothing message passing, reusing increment 1's suspend/resume machinery for cooperative blocking.

API

• task_send of [id, value] — deep-copy + append to id's mailbox, wake it if it waits in task_recv. Send-to-dead (finished/unknown target) is a silent drop returning 0, not an error — Akka dead-letters / Erlang cast, per the prior-art ruling (an error path here would be a nondeterminism magnet). Never blocks (unbounded v1).
• task_recv of null — next message, or block cooperatively on an empty mailbox (suspends via the 1b path; delivered to the stack top on resume, same placeholder mechanism as join). Arena / nested-eval guard like the other suspending builtins.
• task_try_recv of null — non-blocking; next message or null.
• task_kill of id — deterministic teardown: drop mailbox + suspended slice, wake any joiner with an interrupt error, mark dead.

Two bugs found and fixed during share-nothing testing

• task_apply_recv_result fired on any resume with a non-empty mailbox — wrongly draining a message when a task resumed from a plain task_yield. Now gated strictly on recv_blocked.
• The recv wake cleared recv_blocked, so the resume delivery (which needs it set) stopped firing. Now task_deliver leaves it set and makes the wake-enqueue idempotent via mbox_count==1 (first message to a blocked-on-empty task) — a second send before resume can't double-enqueue the task.

Verification

• Release 2614/2614; ASan+UBSan detect_leaks=1 2614/2614 tally 0; test_tasks (now 34 checks) valgrind-clean
• New checks: producer/consumer FIFO delivery (total 100), task_try_recv non-blocking (null/msg/null), send-to-dead drop, deep-copy-message isolation (received value is a copy, not aliased to a post-send mutation), task_kill → joiner interrupt, determinism (byte-identical across runs)
• New executable SPEC mailbox example (byte-exact doc gate); BUILTINS/CHANGELOG updated; doc-drift + freestanding + jit-smoke green

Not in this PR

Virtual time (task_sleep, discrete-event clock) and the pluggable seeded-strategy hook for the liferaft-DST consumer — the remaining #408 increments. Then the differential proof (liferaft's chaos scenario byte-equal on the task layer) and the full determinism-verification sweep.

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