fix: allow skipping client-side fragment validation

[JustinBeckwith]

Summary

• add --skip-fragment / skipFragment for CLI and config users
• add fragmentsToSkip regex/function support to the API
• report matching fragment checks as SKIPPED while still checking the fragmentless HTTP URL
• document the behavior and cover SPA state, hash routers, encoded fragments, strict non-matches, repeated CLI flags, and config loading

Root cause

HTTP requests never send URL fragments to the server. Linkinator can verify traditional fragments against server-rendered HTML, but it cannot infer whether JavaScript will consume a fragment as application state. Treating every missing static identifier as definitely broken creates false positives for links such as TypeScript Playground #code/... URLs.

This change keeps strict fragment validation as the default and adds an explicit, fragment-specific escape hatch. Unlike --skip, it skips only fragment validation; the underlying URL is still requested and must pass.

Validation

• npm run build
• npm run lint (passes; existing Biome schema-version informational notice remains)
• focused CLI/API/config/core tests: 138 passed
• full suite on current Node: 250 passed
• full suite on Node 22: 250 passed
• full suite on Node 26: 250 passed
• coverage suite: 250 passed; 94.01% statements / 88.9% branches / 97.95% functions / 93.95% lines
• npm pack --dry-run
• real tarball install plus installed CLI version/fragment checks
• all standalone binaries built; macOS binary version/fragment checks passed
• npm run docs-test: 17 live README links passed
• live issue reproduction on TypeScript docs:
  • strict mode: 15 fragment failures
  • skipping ^code/ and ^show-examples$: only unrelated GitHub #readme remained strict/broken
  • adding explicit ^readme$: 0 broken; client-state fragments skipped; TypeScript Playground and GitHub base URLs independently remained HTTP-OK

Closes #858