feat(tasks): task_self — a task can learn its own id (#526)

[InauguralPhysicist]

Closes #526

What

task_self of null → the running task's id (a number, in the same integer space task_spawn returns; the main task is 0, including before any task has been spawned).

Why

Surfaced by lib/supervise (#409): task_spawn returns the child's id to the parent, but a task had no way to learn its own id — so a worker could not hand out a reply address, and the message-link supervision pattern (a worker delivering its exit as an ordinary message to its supervisor's mailbox) was not expressible.

Design

• Deterministic by construction: reads pure scheduler state (s->current), no nondeterminism source, so zero tape participation — same contract as task_now.
• No scheduler yet ⇒ 0 (main), consistent with task_virtual_now's no-scheduler behavior.
• Extend-vm checklist: plain builtin (no opcode/AST/CallFrame change, no trace wrap needed).

Tests

tests/test_tasks.eigs (+4 asserts, 69/69):

• main's task_self is 0 with the scheduler active
• a worker's task_self equals the id its spawner got from task_spawn
• the reply-address pattern end-to-end: spawner passes task_self down, worker task_sends its own task_self back, spawner task_recvs it

Docs: docs/BUILTINS.md row, docs/SPEC.md Cooperative tasks (reply-address paragraph), CHANGELOG. Discoverability gate (stdlib_index_check.sh) green: 212 builtins documented.

Validation

• Full suite, release build: 2703/2703 passed
• Full suite, ASan + detect_leaks=1: 2703/2703 passed, leak tally 0 (unchanged)

Follow-up unblocked

A link-based mode for lib/supervise (worker exit delivered as a message) is now expressible.

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