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

Resolves the build half of sow-tools#60 and closes the CLI gap sow-tools#53's
sweep found: PR #71 shipped #53's original surface, not the one #56, #62 and
#65 settled.

- `depot.KeyStore` (`ProbeKey`/`PutReader`/`GetKey`) addresses the zone by
  object key instead of by depot sha, reusing the existing IPv4-pinned
  transport, retry and tri-state probe. The sha-addressed `Backend` now rides
  on it; no new HTTP client.
- `nwsync` gains a sink: the zone by default, a local tree with `--out DIR` as
  the conformance path. Blobs upload as they are produced, the index lands
  last, and a blob already in the zone is skipped without paying for
  compression.
- CLI is now `emit [--as NAME] [--out DIR] <artifact-key> <file>` and
  `assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...`.
  Indexes live beside their artifact with the extension replaced, derived in
  one place. Flags may follow positionals, which cost a run during sow-tools#59.
- Fail-closed: an artifact key whose digest does not match the file is refused,
  and `assemble` refuses an artifact with no index rather than publishing a
  manifest missing a hak.

Conformance re-checked through the new CLI against upstream nwn_nwsync_write
2.1.2 on sow_vfxs_01.hak: the manifest is still byte-identical.

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

internal/depot/remote.go

@@ -10,7 +10,6 @@ import (
 	"net/http"
 	"os"
 	"path/filepath"
-	"strings"
 	"time"
 )
 
@@ -71,16 +70,7 @@ type httpBackend struct {
 
 func (b *httpBackend) Name() string { return b.name }
 
-func (b *httpBackend) storageURL(sha string) string {
-	host := b.cfg.StorageHost
-	// StorageHost is normally a bare host ("storage.bunnycdn.com"); allow a
-	// full scheme (used by tests against httptest.NewServer) to pass through
-	// unchanged.
-	if strings.Contains(host, "://") {
-		return fmt.Sprintf("%s/%s/%s", strings.TrimSuffix(host, "/"), b.cfg.StorageZone, BlobKey(sha))
-	}
-	return fmt.Sprintf("https://%s/%s/%s", host, b.cfg.StorageZone, BlobKey(sha))
-}
+func (b *httpBackend) storageURL(sha string) string { return b.keyURL(BlobKey(sha)) }
 
 func (b *httpBackend) cdnURL(sha string) string {
 	return fmt.Sprintf("%s/%s", b.cfg.CDNBase, BlobKey(sha))
@@ -140,7 +130,8 @@ func (b *httpBackend) Probe(ctx context.Context, sha string) (ProbeState, bool,
 	return b.rangeProbe(ctx, b.storageURL(sha), map[string]string{"AccessKey": b.cfg.ReadKey})
 }
 
-// Put uploads src for sha. cdn is read-only.
+// Put uploads src for sha. cdn is read-only. A depot object is named after the
+// sha256 of its own bytes, so the key's sha doubles as the Checksum header.
 func (b *httpBackend) Put(ctx context.Context, sha, src string) error {
 	if b.name == "cdn" {
 		return errors.New("cdn backend is read-only")
@@ -157,30 +148,7 @@ func (b *httpBackend) Put(ctx context.Context, sha, src string) error {
 		return nil
 	}
 
-	f, err := os.Open(src)
-	if err != nil {
-		return err
-	}
-	defer f.Close()
-
-	req, err := http.NewRequestWithContext(ctx, http.MethodPut, b.storageURL(sha), f)
-	if err != nil {
-		return err
-	}
-	req.Header.Set("AccessKey", b.cfg.WriteKey)
-	req.Header.Set("Checksum", strings.ToUpper(sha))
-
-	resp, err := b.client.Do(req)
-	if err != nil {
-		return err
-	}
-	defer resp.Body.Close()
-	_, _ = io.Copy(io.Discard, resp.Body)
-
-	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
-		return fmt.Errorf("bunny put %s: unexpected status %d", sha, resp.StatusCode)
-	}
-	return nil
+	return b.putFile(ctx, BlobKey(sha), src, sha)
 }
 
 // Get fetches sha into dest via temp file + rename, re-hashing and deleting