Runtime: replace blocking shell setTimeout with event-loop timers

Tracked by: #134 and #246 Related infrastructure: #330

Problem

The current global setTimeout is a test-shell compatibility shim that sleeps the executing agent, then queues a Promise reaction. It has no timer handle, clearTimeout, interval semantics, ref/unref ownership, or real event-loop scheduling. #330 needs an unrefed internal timer lane for AbortSignal.timeout, but silently treating that focused internal record as a complete host timer API would leave the public globals misleading.

Scope

• Replace the blocking shim with context-owned monotonic event-loop timer queues.
• Implement stable timeout/interval handles, clearTimeout/clearInterval, ordering, cancellation, reentrancy, and overflow rules.
• Define refed keepalive versus unrefed work explicitly for shell, C API, worker, and shared-realm execution.
• Run callbacks only on the owning JS event loop; background wakeups may publish readiness but must not execute JS.
• Integrate timer roots/finalizers with precise GC and realm teardown.
• Add fake-clock or bounded deterministic tests instead of timing-sensitive sleeps where practical.

Acceptance

• Calling setTimeout never blocks the executing JavaScript agent.
• Cancellation and intervals have web/Node-compatible observable ordering and exact-once ownership.
• Timer callbacks participate in normal task/microtask checkpoints and keepalive semantics.
• Focused tests and a descendant full CI matrix are green.

#330 may land its deliberately unrefed AbortSignal timer record first; this issue owns the complete public host timer API rather than expanding that ABI slice implicitly.

Triage after #330 established the reusable scheduler constraints:

• General timers must be a separate Context-owned lane; the AbortSignal record remains deliberately unrefed and ABI-shaped.
• A scheduled timeout/interval must own precise roots for its callback and arguments even when its JS handle is dropped. unref() changes shell keepalive only, never callback lifetime.
• Timer selection will be deadline then insertion order. Each callback is one task, followed by a microtask checkpoint before another due timer is selected (the ordering primitive landed in 19d10631).
• Cancellation during a running callback must leave the record valid until callback unwind and suppress interval rescheduling.
• Main, worker, and shared-realm timers need an owning event-loop token; no background thread may execute JS.
• Refed timers may park only in the host keepalive tail. Scheduling itself must return immediately; unrefed timers run only at later host checkpoints.
• Handles will remain inspectable after fire/cancel without retaining a dangling native pointer; active records stay Context-owned and teardown drains them exactly once.

I am starting implementation after #330's descendant matrix is accepted so another push does not cancel that gate.

Implementation is now on main in b5ee60ad with documentation in 2edd8eda.

Delivered:

• nonblocking Context-owned monotonic timeout/interval records, separate from the AbortSignal ABI lane;
• stable Node-style handles (ref, unref, hasRef, refresh, close, numeric coercion) and both clear globals;
• deadline/insertion ordering, one task plus microtask checkpoint per callback, cancellation/reentrancy, and uncaught-exception dispatch;
• precise callback/argument roots with concurrent-mark insertion barriers;
• refed keepalive and unrefed later-checkpoint behavior across shell/module, workers, and shared-realm Thread event loops;
• owner-specific teardown, including dropped unrefed Thread timers;
• concise docs at docs/timers.md and a README link.

Focused normal and TSan timer suites are green in arena, precise-GC, GIL, and no-GIL modes. Awaiting the descendant full matrix before closure.

Acceptance run advanced to descendant head bd045c9c so one full matrix validates both the timer implementation and the next CI-only profile gate: https://github.com/zig-utils/zig-js/actions/runs/29708347319. Before that advance, all four ordinary unit shards (including the new timer tests), docs, test262, ABI, benchmark, feature-profile, SIMD, and no-GIL witness jobs were green with no failures; focused timer TSan was already green locally.

The non-blocking context-owned timer implementation landed in b5ee60ad, its docs/ownership follow-ups landed in 2edd8eda and bd045c9c, focused local TSan was green, and descendant full CI completed with 42 successes, one intentional nightly skip, and zero failures/cancellations: https://github.com/zig-utils/zig-js/actions/runs/29716788619.