test(git): cross colon-flag clusters with real blobs in the differential fuzzer

The blocker hid for three rounds because the fuzzer only ever paired a colon-carrying
flag cluster (`-wG x:y`, `-pS url:1`, …) with a blob-LESS `HEAD` (the CommitMisroute
trap). It never placed such a cluster BEFORE a real blob, which is exactly where the
walker gap bit.

Adds four crossed kinds:
* ClusterLargeUtf8 — cluster + large UTF-8 blob → MUST window (0 silent loss);
* ClusterLatin1 — cluster + Latin-1 blob → byte-identical to `git show <same args>`,
  no U+FFFD mojibake;
* TextconvLargeUtf8 / PathspecLargeUtf8 — `--textconv` or a trailing `-- <path>` →
  never windowed, byte-identical passthrough.

Plus a dedicated end-to-end test that `-wG x:y HEAD:large.txt` windows and that its
recovery hint (which names only the blob arg) reconstructs the blob byte-for-byte. The
extended fuzzer FAILS on the pre-fix code (verified: cluster windowing regression +
Latin-1 fidelity break) and passes on the fix. Seed and iteration count unchanged.