Make lint and type checks strict and non-mutating

[stefan-jansen]

Findings: BR-004, BR-011.

Make CI lint inspect the committed checkout without fixing it and enable correctness-relevant type
categories. Ignores must be narrow, justified, and limited to verified third-party typing defects.

Primary surfaces: Ruff and ty configuration, CI commands, 18 exposed diagnostics, public typing
tests.

Acceptance:

• A lint violation returns nonzero and leaves the checkout unchanged.
• Invalid argument, unresolved attribute, invalid return, and unresolved import categories are not
  globally suppressed.
• Strict ty check passes production source and representative installed-wheel consumers.

Dependencies: Milestones 1-2.

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Plan reference: /home/stefan/ml4t/libraries/ml4t-backtest-dev/.workspace/work/backtest-stable-release/milestone-3/plan.md - Issue 15 in milestone 0.1.0-m3 - Compatibility and release enforcement.