docs: Explain how schema validation works

[Copilot]

No dedicated guide existed explaining the internals of schema validation — how rules are collected, evaluated, and composed into a pass/fail decision.

Changes

• docs/guides/features/schema-validation.md (new): covers the full validation pipeline (schema matching → rule evaluation → row aggregation), the four rule types (column-level, check parameter, primary key, schema-level @dy.rule() and group rules), null handling semantics, and a comparison table of validate / filter / is_valid / cast.
• docs/guides/features/index.md: registers the new page in the feature toctree.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (72fb1a6) to head (948eb0e).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           56        56           
  Lines         3218      3218           
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  Hits          3218      3218           

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