feat: support relative and multiple diagnoses for narrow

[gjbkz]

Summary

• add relative paths and optional shared-issue typing to narrowing diagnoses
• preserve lazy first-issue and ordered all-issues validation semantics
• document and test nested object/array path prefixing and edge cases

Tests

• npm run lint
• npm run test:type
• npm run test:unit
• npm run test:example
• npx c8 npm run test
• bun test src --coverage --coverage-reporter=lcov
• npm run build
• ESM and CommonJS package-entry smoke tests

Closes #438

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.74%. Comparing base (7dfba76) to head (ae11029).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #445   +/-   ##
=======================================
  Coverage   97.73%   97.74%           
=======================================
  Files          50       50           
  Lines        2698     2703    +5     
  Branches      292      293    +1     
=======================================
+ Hits         2637     2642    +5     
  Misses         61       61           

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