fix(isMagnetURI): restore case-insensitive scheme matching

[FanWu-ai]

The component regex accepts case-insensitive magnet schemes, but the prefix check introduced in #1992 rejects uppercase and mixed-case schemes before reaching it. Restore the previous behavior with an anchored case-insensitive prefix check, consistent with RFC 3986 §3.1.

The regression covers direct/later exact-topic parameters, BitTorrent v1/v2, and malformed-prefix controls. Query data and existing component validation are unchanged.

Validation on Node 24.19.0:

• Pristine npm test: 323 passing.
• Regression before fix: 323 passing, 1 failing.
• Final npm test: 324 passing; all builds/minification, ESLint and coverage passed.
• Coverage unchanged: 100% statements/functions/lines, 96.59% branches.
• 4,992 cross-distribution assertions across six entry points and 768 additional scheme/prefix cases passed; git diff --check passed.
• Other Node versions were not run locally; hosted checks are reported separately.

Checklist:

• Source and regression changes only; generated files excluded.
• README reviewed; existing contract unchanged.
• Regression added and full test command passed.
• RFC 3986 §3.1 and historical PR feat(isMagnetURI): support Bittorrent v2 #1992 referenced.

Prepared with OpenAI Codex assistance.