feat(nwsync): emit blobs in parallel with a bounded worker pool (#79)

Emit is latency-bound, not CPU-bound. Every blob costs two serial HTTP
round-trips to the zone — an existence probe, then an upload — so a hak
with a few thousand resources pays a few thousand serialised latencies.
A measured backfill spent 26 seconds of CPU across 9.5 minutes of wall
clock, on a host with three of four cores idle and 5 GB free.

Emit now hashes, compresses and stores `--jobs N` resources at once
(default 16, matching DEPOT_JOBS and the transport's idle connections
per host).

Three properties had to survive, and each has a test:

- The manifest's bytes are promised deterministic by emitterVersion, so
  entries is index-addressed rather than appended to: a worker owns
  entries[i] alone and the slice comes back in artifact order whatever
  order the uploads finish in.
- The index is still the publication marker, so any worker's failure
  aborts the run before a manifest is written. Workers drain the rest of
  the channel instead of returning, which keeps the feeder from blocking
  on workers that have gone away.
- Two resrefs holding identical bytes still share one blob. Serially the
  sink's existence check absorbed that; in parallel both workers would
  probe, both miss, and both upload. Claiming the sha1 in-process
  restores the dedupe and skips a probe round-trip as well.

Peak memory is now the resources in flight rather than one resource, so
the ceiling is N times the 15 MB per-resource limit and its compressed
copy — bounded by a constant this package enforces itself, and still
nowhere near tracking the archive.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/command-surface.md

@@ -48,10 +48,19 @@ beside the artifact itself with the extension replaced, so `emit` and
 `assemble` agree on where it is without being told.
 
 ```
-nwsync emit     [--as NAME] [--out DIR] <artifact-key> <file>
+nwsync emit     [--as NAME] [--out DIR] [--jobs N] <artifact-key> <file>
 nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
 ```
 
+`emit` is latency-bound, not CPU-bound: every blob costs an existence probe
+plus an upload, and a measured backfill spent 26 seconds of CPU across 9.5
+minutes of wall clock. `--jobs N` (default 16) sets how many resources are in
+flight at once. The manifest is byte-identical at any value — the number of
+workers is never observable in the output. Peak memory is `N` times the
+per-resource limit of 15 MB plus its compressed copy, so raising `N` far past
+the default costs real memory for little gain: the transport keeps 16 idle
+connections per host, and past that a worker pays a fresh TLS handshake.
+
 Both verbs upload by default; nothing bulky is ever written to the runner's
 disk. `--out DIR` writes a local repository tree instead, which is the
 conformance path against upstream `nwn_nwsync_write`. The zone comes from