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/run.go

@@ -12,12 +12,17 @@ import (
 	"flag"
 	"fmt"
 	"io"
+	"os"
 )
 
 const (
 	exitOK       = 0
 	exitUsage    = 64
 	exitInternal = 70
+	// exitDrift says the command worked and the zone is wrong, which is a
+	// different thing for CI to act on than the command failing. It matches
+	// depot's code for the same meaning.
+	exitDrift = 1
 )
 
 // Run executes an nwsync subcommand. args[0] is the subcommand (emit|assemble);
@@ -32,6 +37,8 @@ func Run(args []string, stdout, stderr io.Writer) int {
 		return runEmit(args[1:], stderr)
 	case "assemble":
 		return runAssemble(args[1:], stderr)
+	case "verify":
+		return runVerify(args[1:], stdout, stderr, os.Getenv)
 	case "-h", "--help", "help":
 		printRunUsage(stdout)
 		return exitOK
@@ -44,17 +51,27 @@ func Run(args []string, stdout, stderr io.Writer) int {
 
 func printRunUsage(w io.Writer) {
 	fmt.Fprint(w, `usage:
-  nwsync emit     [--as NAME] [--out DIR] <artifact-key> <file>
+  nwsync emit     [--as NAME] [--out DIR] [--verify] <artifact-key> <file>
   nwsync assemble --group-id N [--tlk-key KEY] [--out DIR] <artifact-key>...
+  nwsync verify   [--sample N] [--base URL] <manifest-sha1>
 
 emit explodes one .hak/.erf or one loose file (the TLK) into NWSync blobs plus
 a NSYM index covering only that artifact, and uploads both. assemble merges
 those indexes into one manifest, reading no bulk data. Artifact keys are depot
 keys; an index lives beside its artifact, with the extension replaced.
 
+verify reads a published manifest and its blobs back through the public pull
+zone, with no credential, and decompresses and hashes every one. It is the only
+check on a published blob upstream of a player's client.
+
+--verify makes emit hash what it would otherwise skip. emit normally treats a
+blob's presence as proof of its contents, so without this an object written
+truncated, or written by an emitter since found broken, is skipped forever.
+
 --out DIR writes to a local repository tree instead of uploading, which is the
 conformance path against upstream nwn_nwsync_write. Without it, the zone comes
 from NWSYNC_STORAGE_ZONE, NWSYNC_STORAGE_PASSWORD and BUNNY_STORAGE_HOST.
+verify needs none of those; its base comes from --base or NWSYNC_PULL_BASE.
 `)
 }
 
@@ -82,6 +99,7 @@ func runEmit(args []string, stderr io.Writer) int {
 	as := fs.String("as", "", "published name of the artifact, when it differs from the key")
 	out := fs.String("out", "", "write to a local repository tree instead of uploading")
 	jobs := fs.Int("jobs", defaultEmitJobs, "resources to hash, compress and store at once")
+	verify := fs.Bool("verify", false, "read back and hash blobs that already exist instead of trusting their presence")
 	positional, err := parseArgs(fs, args)
 	if err != nil {
 		return exitUsage
@@ -101,6 +119,7 @@ func runEmit(args []string, stderr io.Writer) int {
 		As:           *as,
 		OutDir:       *out,
 		Jobs:         *jobs,
+		Verify:       *verify,
 	})
 	if err != nil {
 		fmt.Fprintf(stderr, "nwsync emit: %v\n", err)
@@ -111,6 +130,40 @@ func runEmit(args []string, stderr io.Writer) int {
 	return exitOK
 }
 
+func runVerify(args []string, stdout, stderr io.Writer, getenv func(string) string) int {
+	fs := flag.NewFlagSet("verify", flag.ContinueOnError)
+	fs.SetOutput(stderr)
+	base := fs.String("base", getenv("NWSYNC_PULL_BASE"), "pull zone base URL to read through")
+	sample := fs.Int("sample", 0, "check this many random blobs instead of all of them")
+	jobs := fs.Int("jobs", defaultEmitJobs, "blobs to fetch and hash at once")
+	positional, err := parseArgs(fs, args)
+	if err != nil {
+		return exitUsage
+	}
+	if len(positional) != 1 {
+		fmt.Fprintf(stderr, "nwsync verify: exactly one <manifest-sha1> is required\n")
+		return exitUsage
+	}
+
+	result, err := Verify(VerifyOptions{
+		ManifestSHA1: positional[0],
+		Base:         *base,
+		Sample:       *sample,
+		Jobs:         *jobs,
+		Log:          stderr,
+	})
+	if err != nil {
+		fmt.Fprintf(stderr, "nwsync verify: %v\n", err)
+		return exitInternal
+	}
+	fmt.Fprintf(stdout, "verified %d of %d blobs behind %d resources: %d failures, %d bytes checked\n",
+		result.Checked, result.Blobs, result.Entries, result.Failures, result.Bytes)
+	if result.Failures > 0 {
+		return exitDrift
+	}
+	return exitOK
+}
+
 func runAssemble(args []string, stderr io.Writer) int {
 	fs := flag.NewFlagSet("assemble", flag.ContinueOnError)
 	fs.SetOutput(stderr)