docs: Add schema validation feature guide

[Copilot]

No dedicated guide explained the schema validation pipeline in depth — only the quickstart covered basics.

New guide: docs/guides/features/schema-validation.md

• How rules are applied — column-level (zip_code|min_length), primary key uniqueness, and custom @dy.rule() decorators, all evaluated in a single polars pass
• Three validation methods — validate (strict, raises ValidationError), filter (soft, returns FailureInfo), is_valid (boolean, never raises)
• Error taxonomy — SchemaError (structural: missing/wrong-dtype columns) vs ValidationError (content: rule violations); is_valid silently absorbs both
• Cast behaviour — cast=True attempts dtype coercion before rules run; failed casts surface as <column>|dtype entries in FailureInfo
• Inspecting failures — covers all FailureInfo methods: counts, invalid, details (per-row valid/invalid/unknown breakdown), and cooccurrence_counts
• Superfluous columns — columns absent from the schema are silently dropped from validate/filter output

Also adds the guide as the first entry in docs/guides/features/index.md.

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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 (0f794a9).

Additional details and impacted files

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