feat(contracts): enforce stage record invariants

[sigilmakes]

What

• enforce public stage-record invariants at construction
• centralize finite-number, grid, shift, confidence, cutoff, smearing, vdW, path, and feature-vector validation
• prevent mutable grid/shift aliases and duplicate generated paths crossing stage boundaries
• make JSON conversion reject unsupported/non-finite values and key collisions
• represent open-ended k-mesh schedule bounds as JSON-safe null
• add custom-backend containment and adversarial contract coverage

Stack

• Base: docs/55-target-code-adapter (draft PR docs(architecture): define target-code adapter boundary #58)
• Depends on docs(architecture): define target-code adapter boundary #58 and ultimately PR feat: built-in default QRF k-point model with HuggingFace resolution #50.
• Provides shared invariants for the bundle, pseudo, and QRF follow-ups.

How to test

• uv run pytest -q — 257 passed
• uv run ruff check src tests
• uv run ruff format --check src tests
• uv run pre-commit run --all-files

Closes #56.

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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This draft has been superseded by the consolidated cleanup PR #68.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.