refactor: share comment scanners across detection and mapping

[steipete]

What Problem This Solves

Project detection and language mappers maintained identical copies of the XML, line-comment, and Swift nested-comment scanners. Fixing either copy could leave detection and mapping inconsistent.

Why This Change Was Made

Move the existing implementations unchanged into one source-comment module and import them at each call site. No CLI behavior or runtime floor changes.

User Impact

Maintenance-only refactor; existing feature identities and discovery behavior are preserved.

Evidence

• pnpm typecheck, pnpm lint, pnpm format:check, pnpm test, and pnpm build passed; 941 tests passed, 2 existing platform skips.
• Isolated Codex autoreview: scoped-clean at P0–P2.
• Live proof: invoked the rebuilt node dist/cli.js --root <synthetic-fixture> init --json --quiet and map --json --quiet. The fixture combines nested Swift comments, a commented-out executable declaration, and XML-commented .NET test metadata. The CLI mapped 4 features, including Swift executable Active; it did not map the commented executable or a spurious .NET test suite. Detection returned Swift/C#, SwiftPM/.NET, swift build, and no test command.

[clawsweeper]

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[clawsweeper]

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I am starting a fresh review of this pull request: refactor: share comment scanners across detection and mapping This is item 1/1 in the current shard. Shard 0/1.

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