feat(stdlib): Jobs, Promises & opt-in Sync concurrency (#14)

A layered concurrency library, safe by default. The recommended tier is
Job.* / Promise.*: a Job is a future — Job.run(kind, arg) starts a
cooperative background compute that advances each Job.pump(budget) and
finishes after enough frames (heavy work spreads out instead of hitching),
or Job.defer + Job.fulfill/fail/cancel drives one by hand. Poll with
done/ok/failed/cancelled, read result/error, count outstanding work with
Job.pending. Promise.all/race combine handle lists into a group job resolved
on the main thread; Promise.count_done/all_done power a loading bar.

The advanced, opt-in Sync.* tier (mutex/atomic/channel + cpu_count) is the
"here be dragons" surface for engine-level message passing.

The whole thing is a deterministic cooperative scheduler: results are
collected on the main thread and a Job never touches the ECS world, so
lockstep and replays stay bit-exact — same jobs + same budget reproduce
byte-for-byte on every target, and a preemptive OS-thread backend can slot
behind this same API later. Ludic has no closures, so a Job carries a
compute kind + int arg (or a hand-driven defer) rather than fn()->…, and
Promise progress is polled rather than chained through then.

Written in Ludic and spliced on demand (like Regex/Dict/Numeric): a program
that never mentions Job.*/Promise.*/Sync.* compiles byte-identically and the
C-free bootstrap fixpoint is untouched. New: runtime/native/jobs.ludic,
emit_ns_call dispatch, parse-time splice, examples/library/jobs.ludic (31
self-asserting checks), 33 docs pages + inventory, changeset.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/language/job/_section.md

@@ -0,0 +1,7 @@
+---
+id: job
+title: Job
+order: 37
+---
+
+Background work that stays out of the frame. A <code>Job</code> is a future — a handle to a result that lands later. Kick one off with <code>Job.run</code> (a background compute that advances a little each <code>Job.pump</code> and finishes after enough frames, so heavy work never hitches) or <code>Job.defer</code> (a future you resolve yourself with <code>Job.fulfill</code> / <code>Job.fail</code>). Poll it with <code>done</code> / <code>ok</code> / <code>failed</code> / <code>cancelled</code>, read <code>result</code> / <code>error</code>, and always collect on the main thread — a Job must never touch the ECS world directly. The scheduler is deterministic and cooperative, so the same jobs and the same budget reproduce byte-for-byte, every run and every target. Arguments are positional. Spliced in only when a program mentions <code>Job.*</code>.