fix(nwsync): close the review findings on the framing fix and verify

- compressBlob now asserts its own output declares a content size, instead of
  trusting that the only way to lose the field is the one #86 found. An encoder
  upgrade that finds another way would otherwise republish the same undecodable
  blob in silence, and a blob is skipped by every later emit once written.
- inspectBlob asserts the frame whenever one is present, rather than only when
  the payload is non-empty.
- dirSink stats instead of reading when not verifying. Reading every existing
  blob back on the default path was a plain regression, and it contradicted the
  documented promise that verifying is a repair pass, not the default.
- The manifest sha1 is now checked as hex before it is interpolated into a URL
  path, rather than only measured.
- One blobKey rule for where a blob lives, replacing three copies of the
  fanout-path expression.
- Renamed frameSizeThreshold to oneByteContentSizeCeiling, which says whose
  threshold it is.
- The read-back in zoneSink reads the storage API on purpose; said so, and
  corrected a comment that claimed a failed read-back re-uploads when it
  aborts.
- Tests derive their expected blob counts from the run instead of hard-coding
  fixture arithmetic, and the size-mismatch category has a test.

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

internal/nwsync/manifest.go

@@ -7,6 +7,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"io"
+	"path"
 	"path/filepath"
 	"sort"
 	"strings"
@@ -194,7 +195,14 @@ func marshalSidecar(sidecar Sidecar) ([]byte, error) {
 	return append(body, '\r', '\n'), nil
 }
 
-// blobPath is the data store path for a blob, hash tree depth 2.
+// blobKey is where a blob lives in a zone, hash tree depth 2. emit writes it,
+// verify reads it and the game client requests it, so the rule lives here and
+// nowhere else.
+func blobKey(sha1Hex string) string {
+	return path.Join("data", "sha1", sha1Hex[0:2], sha1Hex[2:4], sha1Hex)
+}
+
+// blobPath is the same location inside a local repository tree.
 func blobPath(root, sha1Hex string) string {
-	return filepath.Join(root, "data", "sha1", sha1Hex[0:2], sha1Hex[2:4], sha1Hex)
+	return filepath.Join(root, filepath.FromSlash(blobKey(sha1Hex)))
 }