test: add synthetic, committable e2e fixtures (base/delta app + opt)

Forge three small fixtures with no proprietary content so the e2e test runs by
default and covers the full decrypt+extract path, including the delta/base VHD merge:

- TEST_T001_..._0.opt              encrypted exFAT (OPTION key)
- TEST_1.00.00_..._0.app           base APP: outer NTFS -> internal_0.vhd (fixed) -> inner NTFS
- TEST_1.01.00_..._1_1.00.00.app   delta APP: differencing internal_1.vhd, auto-merged with base by GUID

The APP fixtures use the synthetic game id TEST, decrypted via the committed
TEST.bin external key (also exercising that key-file fallback). e2e.rs copies the
fixtures + key into a temp dir and runs from there so the delta finds its base and
the key. .gitignore whitelists only these synthetic files so real containers
dropped in fixtures/ stay private. The generator is kept local, not committed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

fixtures/README.md

@@ -0,0 +1,34 @@
+# Test fixtures
+
+These are **synthetic** SEGA fscrypt containers used by `tests/e2e.rs`. They
+contain only dummy files (a few text/XML/stub binaries), so they are safe to
+commit — there is no proprietary game content inside.
+
+| File | Type | What it exercises |
+|------|------|-------------------|
+| `TEST_T001_20240101120000_0.opt` | OPTION | exFAT decrypt + extract (built-in OPTION key) |
+| `TEST_1.00.00_20240101120000_0.app` | APP (base) | outer NTFS → `internal_0.vhd` (fixed VHD) → inner NTFS extract |
+| `TEST_1.01.00_20240102120000_1_1.00.00.app` | APP (delta) | differencing `internal_1.vhd`, auto-merged against the base by VHD GUID |
+| `TEST.bin` | key | external key (16-byte key + 16-byte IV) for game id `TEST` |
+
+The delta is a real differencing VHD linked to the base; decrypting it (with the
+base alongside) merges the two and yields the base files plus the delta's added
+`data/patch_notes.txt` and a modified `readme.txt`.
+
+The `.app` fixtures use the synthetic game id `TEST`, which is **not** in the
+built-in key table — so they are decrypted via the external-key-file fallback
+using the committed `TEST.bin` (this also exercises that fallback path).
+`tests/e2e.rs` copies `TEST.bin` next to the containers and runs from there so
+the binary finds it. The OPTION fixture uses the built-in OPTION key. Everything
+here — keys and filesystem data alike — is synthetic.
+
+You can also drop your own **real** `.app`/`.opt` files in this folder to test
+against them; everything here except these committed fixtures is git-ignored.
+
+## Regenerating
+
+These are generated by a local, uncommitted script set (NTFS/exFAT image
+creation in Docker + a small SEGA fscrypt packer adapted from
+[`beerpsi/x`](https://gitea.tendokyu.moe/beerpsi/x)). The byte output is not
+reproducible (NTFS/exFAT embed creation timestamps), but regenerated fixtures are
+functionally equivalent.