nwsync: upload sink, key-addressed CLI, fail-closed publication marker (#73)

Builds the code half of #60, and closes the CLI gap the #53 sweep found: PR #71 shipped #53's original surface rather than the one #56, #62 and #65 settled.

## What lands

**`depot.KeyStore`** — `ProbeKey` / `PutReader` / `GetKey`, addressing the zone by object key rather than by depot sha. NWSync cannot use the sha-addressed path: a blob is named after the sha1 of its *uncompressed* bytes while the body uploaded is the compressed form, and Bunny's `Checksum` header is sha256 of the body. Per #55 this reuses `internal/depot`'s `httpBackend` — same IPv4-pinned transport, same retry, same tri-state probe — and the sha-addressed `Backend` is now rewritten on top of it. No second HTTP client, no per-instance hash-function fields: the caller passes the key and the checksum, which turned out simpler than #55 expected.

**A sink in `internal/nwsync`** — the zone by default, a local tree under `--out DIR` as the conformance path. Blobs upload as they are produced and the index lands last, so the presence of an index is the publication marker. A blob already in the zone is skipped via #55's probe *without* paying for compression (the body is a thunk) — which matters for the backfill, where compression is the expensive part.

**The settled CLI**

```
nwsync emit     [--as NAME] [--out DIR] <artifact-key> <file>
nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
```

Artifact keys are depot keys; an index lives beside its artifact with the extension replaced (#62), derived in exactly one place so `emit` and `assemble` cannot disagree. Flags may now follow positionals — Go's `flag` stops at the first non-flag argument, which cost a run during #59.

**Fail-closed in two places** — an artifact key whose embedded digest does not match the file is refused (publishing an index under the wrong key silently pairs a manifest with the wrong artifact), and `assemble` refuses an artifact with no index rather than publishing a manifest missing a hak.

## Checks

`make check` green. Six new tests run against a Bunny-shaped `httptest` zone that verifies the `Checksum` header the way Bunny does: blobs-then-index ordering, skip-if-present, no index after a failed upload, key/file mismatch, and assemble reading indexes back out of the zone.

Conformance re-run through the new CLI against upstream `nwn_nwsync_write` 2.1.2 over `sow_vfxs_01.hak` (#59's oracle): the manifest is still **byte-identical**.

## Not in this PR

- **Live upload against the real zone.** The nwsync zone and its credential are #61, still open. Everything here is proven against a fake zone only.
- **Consumer wiring** — #65, in the three producer repos.
- **The mid-hak failure *policy*.** The mechanism is here (fail closed, orphan blobs left, re-run resumes); whether a module release may proceed when an emit failed is a human call, still open on #60.

🤖 Generated with [Claude Code](https://claude.com/claude-code)Reviewed-on: #73

Co-authored-by: vickydotbat <vickydotbat@tutamail.com>

docs/command-surface.md

@@ -43,10 +43,32 @@ flags are mutually exclusive.
 
 `nwsync emit` runs where an artifact is born (a `.hak`/`.erf`, or a loose file
 such as the TLK); `nwsync assemble` runs at module release and reads only the
-small per-artifact manifests. `--order` lists artifact names highest priority
-first: a resref in more than one artifact resolves to the earliest one, the way
-the game resolves it. `--group-id` is per channel — 1 is current, 2 is testing,
-and 0 leaves the field out of the sidecar.
+small per-artifact indexes. Both take **depot keys**: an artifact's index lives
+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 assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
+```
+
+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
+`NWSYNC_STORAGE_ZONE` and `NWSYNC_STORAGE_PASSWORD`, with the host from
+`BUNNY_STORAGE_HOST` — NWSync data is a separate zone from the asset depot.
+
+`assemble`'s artifact keys are in `Mod_HakList` order, highest priority first: a
+resref in more than one artifact resolves to the earliest one, the way the game
+resolves it. `--tlk-key` has its own slot because the TLK shadows nothing.
+`--group-id` is per channel — 1 is current, 2 is testing, and 0 leaves the field
+out of the sidecar.
+
+`emit` uploads blobs first and the index last, so the presence of an index is
+the publication marker: an artifact whose emit died halfway leaves real blobs in
+the zone and no index. Blob names are content hashes, so re-running skips
+whatever already landed, and `assemble` fails closed on an artifact with no
+index rather than publishing a manifest that is missing a hak.
 
 ## Hidden compatibility aliases