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/sync/sync-atomic.md

@@ -0,0 +1,20 @@
+---
+id: sync-atomic
+name: Sync.atomic
+category: sync
+kind: namespace-method
+tokens: Sync.atomic
+sig: Sync.atomic() -> int
+tip: Create an atomic counter (starts at 0).
+order: 4
+ns: Sync
+member: atomic
+---
+
+Create an atomic counter (starts at 0).
+
+Returns an atomic-counter handle. Read it with <code>Sync.get</code>, write with <code>Sync.set</code>, accumulate with <code>Sync.add</code>, and swap conditionally with <code>Sync.cas</code>.
+
+```ludic
+let a = Sync.atomic()
+```