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>

internal/depot/keystore.go

@@ -0,0 +1,121 @@
+package depot
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"os"
+	"strings"
+)
+
+// KeyStore is the zone addressed by object key rather than by depot sha. The
+// depot names every object after the sha256 of its contents; NWSync does not —
+// a blob is named after the sha1 of its *uncompressed* bytes while the body
+// uploaded is the compressed form, and a per-artifact index is named after its
+// artifact. Both addressing modes want the same transport, retry and probe
+// discipline, so the sha-addressed Backend rides on this rather than the other
+// way round.
+type KeyStore interface {
+	// ProbeKey returns the existence state of one key. transient=true means a
+	// retry might change the answer — never read it as "missing, re-upload".
+	ProbeKey(ctx context.Context, key string) (state ProbeState, transient bool, err error)
+	// PutReader uploads size bytes read from r to key. checksum is the
+	// uppercase hex sha256 of those bytes, which Bunny verifies server-side.
+	PutReader(ctx context.Context, key string, r io.Reader, size int64, checksum string) error
+	// GetKey fetches the whole object at key. Small objects only — it holds
+	// the body in memory and does no hash check, because a key is not always
+	// a content hash.
+	GetKey(ctx context.Context, key string) ([]byte, error)
+}
+
+// NewKeyStore returns a KeyStore for cfg's storage zone. Fails closed on a
+// missing host or read key, matching NewBackend.
+func NewKeyStore(cfg Config) (KeyStore, error) {
+	if cfg.StorageHost == "" {
+		return nil, errors.New("storage backend requires a storage host")
+	}
+	if cfg.StorageZone == "" {
+		return nil, errors.New("storage backend requires a storage zone")
+	}
+	if cfg.ReadKey == "" {
+		return nil, errors.New("storage backend requires a read key")
+	}
+	return &httpBackend{name: "bunny", client: newHTTPClient(cfg), cfg: cfg}, nil
+}
+
+// keyURL is the storage URL of one object key.
+func (b *httpBackend) keyURL(key 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, "://") {
+		host = "https://" + host
+	}
+	return fmt.Sprintf("%s/%s/%s", strings.TrimSuffix(host, "/"), b.cfg.StorageZone, key)
+}
+
+func (b *httpBackend) ProbeKey(ctx context.Context, key string) (ProbeState, bool, error) {
+	return b.rangeProbe(ctx, b.keyURL(key), map[string]string{"AccessKey": b.cfg.ReadKey})
+}
+
+func (b *httpBackend) PutReader(ctx context.Context, key string, r io.Reader, size int64, checksum string) error {
+	if b.name == "cdn" {
+		return errors.New("cdn backend is read-only")
+	}
+	if b.cfg.WriteKey == "" {
+		return errors.New("storage backend requires a write key to write")
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPut, b.keyURL(key), r)
+	if err != nil {
+		return err
+	}
+	req.ContentLength = size
+	req.Header.Set("AccessKey", b.cfg.WriteKey)
+	// Bunny defines Checksum as sha256 of the body and rejects a mismatch, so
+	// this is server-side integrity checking, not decoration.
+	req.Header.Set("Checksum", strings.ToUpper(checksum))
+
+	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("put %s: unexpected status %d", key, resp.StatusCode)
+	}
+	return nil
+}
+
+func (b *httpBackend) GetKey(ctx context.Context, key string) ([]byte, error) {
+	resp, err := b.get(ctx, b.keyURL(key), map[string]string{"AccessKey": b.cfg.ReadKey})
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode < 200 || resp.StatusCode >= 300 {
+		_, _ = io.Copy(io.Discard, resp.Body)
+		return nil, fmt.Errorf("get %s: unexpected status %d", key, resp.StatusCode)
+	}
+	return io.ReadAll(resp.Body)
+}
+
+// putFile uploads the file at src to key, streaming it. checksum is the
+// uppercase hex sha256 of the file's bytes.
+func (b *httpBackend) putFile(ctx context.Context, key, src, checksum string) error {
+	f, err := os.Open(src)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	info, err := f.Stat()
+	if err != nil {
+		return err
+	}
+	return b.PutReader(ctx, key, f, info.Size(), checksum)
+}