P1: Split `route-scanner.ts` into route/island/cem scanners

[SisyphusZheng]

What to build

packages/ssg/src/route-scanner.ts (675 lines) mixes route scanning, island scanning, CEM parsing, and package manifest scanning. Split into focused modules.

Acceptance criteria

• New files: route-scanner.ts, island-scanner.ts, cem-scanner.ts.
• Public API preserved.
• All ssg tests pass.

Blocked by

None