fix: correct HuggingFace checkpoint filenames in code and installers

[mountarreat]

What does this change?

The Python auto-download path in CorridorKeyModule/backend.py
and the bundled Windows/Linux install scripts point at
HuggingFace URLs that do not match the filenames actually hosted
on the model repo.

On a fresh clone with an empty CorridorKeyModule/checkpoints/
directory, the Python auto-download path raises:

RuntimeError: Failed to download CorridorKey checkpoint from
https://huggingface.co/nikopueringer/CorridorKey_v1.0.
Check your network connection and try again.
Original error: 404 Client Error. Entry Not Found for url:
https://huggingface.co/nikopueringer/CorridorKey_v1.0/resolve/main/CorridorKey.pth.

Network is fine. The URL is wrong. The error message blames the
network, which sends users looking in the wrong place first.

Running the bundled Install_CorridorKey_Windows.bat or
Install_CorridorKey_Linux_Mac.sh instead produces:

Downloading CorridorKey.safetensors...
curl: (22) The requested URL returned error: 404
safetensors not available yet -- falling back to CorridorKey.pth...

The safetensors file IS available on HF; the curl URL is just
misspelled. Current installer users get silently downgraded to
the legacy .pth format.

Ground truth from HuggingFace's model API
(https://huggingface.co/api/models/nikopueringer/CorridorKey_v1.0):
the repo hosts only CorridorKey_v1.0.safetensors and
CorridorKey_v1.0.pth. The unversioned filenames
CorridorKey.safetensors and CorridorKey.pth do not exist.

Five files fixed, all one-line string changes, six insertions
and six deletions:

• CorridorKeyModule/backend.py:33-34:
  HF_CHECKPOINT_FILENAME_SAFETENSORS and HF_CHECKPOINT_FILENAME
  get the _v1.0 suffix. The .pth constant is also corrected
  by the open PR perf: unblock benchmark harness, add setup docs and perf PR template #221; the two changes are content-identical, so
  the PRs merge cleanly either way around.
• Install_CorridorKey_Windows.bat:64: curl URL for the
  .safetensors download.
• Install_CorridorKey_Linux_Mac.sh:79: curl URL for the
  .safetensors download.
• README.md:78: doc claim that the auto-download saves the file
  as CorridorKey.safetensors, now correctly
  CorridorKey_v1.0.safetensors.
• tests/test_backend.py:219: stale docstring.

A few adjacent items were considered and deliberately left alone:

• The .pth URLs in the installer scripts are already correct
  (CorridorKey_v1.0.pth).
• The installer scripts' local save paths (SAFETENSORS_PATH,
  PTH_PATH) keep their unversioned filenames on disk, so the
  installer's idempotent if exist skip check still hits for
  users who ran the prior installer. _discover_checkpoint globs
  *.safetensors and *.pth, so the local stem does not affect
  runtime behavior.
• Prose references to the filenames in CONTRIBUTING.md,
  docs/LLM_HANDOVER.md, and the convert_pth_to_safetensors.py
  usage docstring describe the file formats generically, not
  specific HF paths.

How was it tested?

• uv run ruff format --check: clean
• uv run ruff check: clean
• uv run pytest -q --tb=short -m "not gpu": 363 passed, 1
  skipped (MLX), 4 deselected (GPU). Same count as upstream.
• HuggingFace API filename listing cross-checked against the
  constants via
  https://huggingface.co/api/models/nikopueringer/CorridorKey_v1.0.
• uv run corridorkey --help: exits 0.

Checklist

• uv run pytest passes
• uv run ruff check passes
• uv run ruff format --check passes