Fix duplicate Civitai weight selection

[PaulFidika]

What changed

• De-duplicate Civitai safetensors entries that share a destination filename, preserving primary-first selection.
• Preserve distinct filenames for genuine multi-file bundles.
• Add regression coverage and bump the package to 0.26.2.

Why

Civitai model version 2884631 advertises an fp16 primary and fp32 alternative under the same filename. The worker planned and downloaded both, overwrote the first with the second, and inflated disk preflight from about 18.2 GiB to 50.5 GiB. This made the live recovery ingest fail on a 10 GiB CPU pod before any CAS publish.

Validation

• uv run --no-sync pytest -q tests/convert/test_civitai_gguf_select.py -k 'not ingest_civitai_gguf_classification' tests/convert/test_download_skip.py — 20 passed, 1 deselected.
• Live Civitai metadata for model version 2884631 selects one primary file: 6,938,041,288 bytes.
• Ruff on both touched Python files, HTTP-timeout guard, git diff --check, and uv build --offline pass.

Existing baseline failures

Untouched origin/master already has one GGUF fixture failure, five mypy errors, and uv sync --locked --extra dev currently fails because the mutable torch==2.13.0+cu130 wheel no longer matches the hashes recorded in uv.lock. This PR does not touch those paths or dependency hashes.