fix(nwsync): declare Frame_Content_Size, and verify what is published (#86, #85)

These two land together on purpose. Fixing the encoder alone changes nothing
for the blobs already in the zone, because emit skips whatever is already
present.

#86 — klauspost/compress omits the zstd Frame_Content_Size field for inputs
under 256 bytes, which the format permits. Reference libzstd never does, so the
NWN client — which sizes its output buffer from ZSTD_getFrameContentSize and has
therefore never met a frame without one — rejected roughly 6% of our blobs
outright. Any single one stops a sync dead, so no client could complete a sync
of the live manifest.

No encoder option changes this, so emit re-headers the affected frames into the
shape libzstd itself emits: Single_Segment_flag set, Window_Descriptor dropped,
and the freed byte spent on a one-byte Frame_Content_Size. Same length in, same
length out. Verified against the zstd CLI end to end: a real emitted 230-byte
blob now reports its decompressed size where it previously reported none.
emitter_version goes to 2, so assemble refuses to merge an index written by the
encoder that omitted the field.

#85 — a published blob was verified by exactly one thing, a player's client, at
download time. `nwsync verify <manifest-sha1>` now reads a manifest and its
blobs back through the public pull zone with no credential, decompresses and
hashes every one, and reports missing / malformed framing / size mismatch / hash
mismatch per blob. `--sample N` makes a routine check cheap against a manifest
that is ~69,000 blobs and 15 GB. `emit --verify` applies the same check where
emit would otherwise trust presence, and replaces a stored blob that is not what
its name claims — which is what makes the #86 blobs repairable.

Both new checks assert the frame property, not just a round trip. Round-tripping
cannot see this class of fault: both the zstd CLI and Go's decoder stream such a
frame happily, being more capable decoders than the client's, which is how the
defect reached production and survived an audit.

Refs #54, #55, #59, #75, sow-platform#79.

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

internal/nwsync/zone_test.go

@@ -21,6 +21,13 @@ type fakeZone struct {
 	objects map[string][]byte
 	puts    []string
 	failOn  func(key string) bool // when true, the PUT fails
+	url     string                // base the same objects are readable at
+}
+
+// pullZone reads the fake zone the way the public pull zone is read: plain
+// unauthenticated GETs, no storage API.
+func (z *fakeZone) pullZone() blobSource {
+	return newPullZone(z.url)
 }
 
 func newFakeZone(t *testing.T) (*fakeZone, func(string) string) {
@@ -28,6 +35,7 @@ func newFakeZone(t *testing.T) (*fakeZone, func(string) string) {
 	zone := &fakeZone{objects: map[string][]byte{}}
 	server := httptest.NewServer(zone)
 	t.Cleanup(server.Close)
+	zone.url = server.URL + "/sow-nwsync"
 	getenv := func(name string) string {
 		switch name {
 		case "NWSYNC_STORAGE_ZONE":