Verify XMODEM downloads against advertised MD5

[righteousgambit]

Closes #666

What changed

• Retain the whole-file MD5 advertised by XMODEM block zero.
• Hash each actual, unpadded payload slice as it is written.
• At EOT, keep the existing ACK behavior but reject the local artifact with None when the advertised digest is absent or does not match.
• Normalize hexadecimal digest comparisons so an uppercase local MD5 still takes the existing cache shortcut.
• Add deterministic tests for a matching transfer, a prematurely terminated/truncated transfer, and the uppercase-local-MD5 shortcut (0, empty output, three CAN writes).
• Document the integrity fix in the changelog.

Protocol compatibility

The receiver continues to ACK EOT on the wire. The behavioral change is local: a transfer whose bytes do not match the machine-advertised MD5 is no longer reported as successful to download_file().

Validation

Pinned develop (2ea66c10e96d88013a04441134d3d4f8e8c88510) against the new tests:

2 failed, 1 passed

The truncated-transfer assertion observes 8192 instead of None; the uppercase-local-MD5 test observes a completed transfer instead of the cache short-circuit.

Corrected implementation:

3 passed

Quality checks:

ruff check carveracontroller/XMODEM.py tests/unit/test_xmodem_download_integrity.py
ruff format --check carveracontroller/XMODEM.py tests/unit/test_xmodem_download_integrity.py
python -m compileall -q carveracontroller/XMODEM.py tests/unit/test_xmodem_download_integrity.py
git diff --check HEAD^ HEAD

Real branch verification

The repository's locked Poetry environment was installed from poetry.lock. The targeted suite passed on real fork commit f96ceccf1615, whose sole parent is pinned upstream develop@2ea66c10e96d88013a04441134d3d4f8e8c88510.

poetry run python -m pytest -q tests/unit/test_xmodem_download_integrity.py

Result: 3 passed.

Quality gates on the real branch:

• Ruff check: passed
• Ruff format --check: passed
• git diff --check 2ea66c10e96d88013a04441134d3d4f8e8c88510..f96ceccf161516a5e02c2eb995b1b7661aba644b: passed