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>

tools/docgen/inventory.json

@@ -437,6 +437,45 @@
     "set-clear",
     "set-members"
   ],
+  "job": [
+    "job-defer",
+    "job-run",
+    "job-fulfill",
+    "job-fail",
+    "job-cancel",
+    "job-pump",
+    "job-done",
+    "job-ok",
+    "job-failed",
+    "job-cancelled",
+    "job-result",
+    "job-error",
+    "job-pending",
+    "job-free"
+  ],
+  "promise": [
+    "promise-all",
+    "promise-race",
+    "promise-count_done",
+    "promise-all_done"
+  ],
+  "sync": [
+    "sync-mutex",
+    "sync-lock",
+    "sync-unlock",
+    "sync-try_lock",
+    "sync-atomic",
+    "sync-get",
+    "sync-set",
+    "sync-add",
+    "sync-cas",
+    "sync-channel",
+    "sync-send",
+    "sync-recv",
+    "sync-can_recv",
+    "sync-len",
+    "sync-cpu_count"
+  ],
   "huge": [
     "huge-from",
     "huge-add",