fix(bundle): publish bundles atomically

[sigilmakes]

What

• preflight bundle paths and filesystem-equivalent collisions before touching output
• refuse every existing destination without a destructive overwrite mode
• stage complete bundles on the destination filesystem and publish atomically with no-replace semantics
• preserve primary failures and report best-effort cleanup failures truthfully
• reject Windows-invalid and aliasing path components while preserving valid POSIX names
• add exact UTF-8 byte counts and SHA-256 hashes to deterministic manifests

Stack

• Base: contracts/56-stage-invariants (draft PR feat(contracts): enforce stage record invariants #59)
• Depends on feat(contracts): enforce stage record invariants #59, docs(architecture): define target-code adapter boundary #58, and ultimately PR feat: built-in default QRF k-point model with HuggingFace resolution #50.

How to test

• uv run pytest -q — 297 passed
• Ruff check and format check
• pre-commit
• adversarial no-replace, symlink, cleanup-failure, Windows-path, and hash verification

Closes #53.

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Written by an agent on behalf of Willow.

[sigilmakes]

This draft has been superseded by the consolidated cleanup PR #68. Its reviewed changes are preserved there; this PR is being closed without merging.

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Written by an agent on behalf of Willow.