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/compressedbuf.go

@@ -3,6 +3,7 @@ package nwsync
 import (
 	"bytes"
 	"encoding/binary"
+	"encoding/hex"
 	"fmt"
 
 	"github.com/klauspost/compress/zstd"
@@ -31,6 +32,18 @@ var (
 	blobDecoder, _ = zstd.NewReader(nil, zstd.WithDecoderConcurrency(1))
 )
 
+// zstd frame header bits we care about. A frame starts with the magic, then a
+// one-byte Frame_Header_Descriptor: bits 7-6 size the Frame_Content_Size field,
+// bit 5 is Single_Segment_flag, bits 1-0 size the Dictionary_ID field.
+const (
+	zstdFrameMagic      = "\x28\xb5\x2f\xfd"
+	frameSingleSegment  = 1 << 5
+	frameDictionaryMask = 0x03
+	// The size below which klauspost/compress writes no Frame_Content_Size,
+	// matching the field's own encoding threshold.
+	frameSizeThreshold = 256
+)
+
 // compressBlob wraps data in NWCompressedBuffer framing.
 func compressBlob(data []byte) []byte {
 	var out bytes.Buffer
@@ -38,10 +51,87 @@ func compressBlob(data []byte) []byte {
 	for _, field := range header {
 		_ = binary.Write(&out, binary.LittleEndian, field)
 	}
-	out.Write(blobEncoder.EncodeAll(data, nil))
+	out.Write(declareFrameContentSize(blobEncoder.EncodeAll(data, nil), len(data)))
 	return out.Bytes()
 }
 
+// inspectBlob unwraps a stored blob the way the game client reads it, and is the
+// only reader that should be trusted to judge a published blob.
+//
+// It asserts the frame property on top of the round trip. Go's decoder — like
+// the zstd CLI — streams a frame that declares no content size, so a check that
+// only decompresses and hashes is a *more* capable decoder than the client's: it
+// certifies exactly the blobs the client rejects, which is how #86 reached
+// production and survived an audit.
+func inspectBlob(blob []byte) ([]byte, error) {
+	data, err := decompressBlob(blob)
+	if err != nil {
+		return nil, fmt.Errorf("malformed framing: %w", err)
+	}
+	if len(data) > 0 && !frameDeclaresContentSize(blob[blobHeaderBytes:]) {
+		return nil, fmt.Errorf("malformed framing: the zstd frame declares no content size, which the game client cannot decode")
+	}
+	return data, nil
+}
+
+// blobMatchesName holds a stored blob to its own file name: a blob is named
+// after the sha1 of its uncompressed bytes, so the name is a complete statement
+// about the contents and nothing else is needed to check it.
+func blobMatchesName(blob []byte, sha1Hex string) error {
+	data, err := inspectBlob(blob)
+	if err != nil {
+		return err
+	}
+	if got := hex.EncodeToString(sha1Sum(data)); got != sha1Hex {
+		return fmt.Errorf("blob %s holds the contents of %s", sha1Hex, got)
+	}
+	return nil
+}
+
+// frameDeclaresContentSize reports whether a zstd frame states how many bytes it
+// decompresses to. A frame with a zero-sized Frame_Content_Size field declares
+// one only when Single_Segment_flag is set; otherwise the size is unknown.
+func frameDeclaresContentSize(frame []byte) bool {
+	if len(frame) < 5 || string(frame[:4]) != zstdFrameMagic {
+		return false
+	}
+	descriptor := frame[4]
+	return descriptor>>6 != 0 || descriptor&frameSingleSegment != 0
+}
+
+// declareFrameContentSize rewrites a frame that does not declare its
+// Frame_Content_Size so that it does, and returns any other frame unchanged.
+//
+// klauspost/compress omits the field for inputs under 256 bytes, which the spec
+// 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 — rejects the blob outright with an empty "potential
+// compression error" (#86). No encoder option changes this, so the frame is
+// re-headered here.
+//
+// The result is the shape libzstd itself emits for the same input: setting
+// Single_Segment_flag drops the Window_Descriptor byte, and the freed byte pays
+// for a one-byte Frame_Content_Size. Window_Size then equals the content size,
+// which is sound because the content is under 256 bytes and every match in it
+// therefore falls inside that window. Same length in, same length out.
+func declareFrameContentSize(frame []byte, size int) []byte {
+	if size <= 0 || size >= frameSizeThreshold || len(frame) < 6 || string(frame[:4]) != zstdFrameMagic {
+		return frame
+	}
+	descriptor := frame[4]
+	// Rewrite only the exact shape a small input produces: no declared size, no
+	// single segment, no dictionary. Anything else either declares a size
+	// already or is not a frame this reinterpretation is safe on.
+	if descriptor>>6 != 0 || descriptor&frameSingleSegment != 0 || descriptor&frameDictionaryMask != 0 {
+		return frame
+	}
+	reframed := make([]byte, len(frame))
+	copy(reframed, frame)
+	reframed[4] = descriptor | frameSingleSegment
+	reframed[5] = byte(size) // replaces Window_Descriptor
+	return reframed
+}
+
 // decompressBlob unwraps NWCompressedBuffer framing. It exists so a blob this
 // package wrote — or one upstream wrote — can be compared by its uncompressed
 // bytes, which is the only comparison that is meaningful across zstd