P1: Split `entry-renderer.ts` into generated-section helpers

[SisyphusZheng]

What to build

packages/ssg/src/entry-renderer.ts (757 lines) builds a single giant code string. Split into helpers for imports, document wrapper, island registration, route registration, middleware, etc.

Acceptance criteria

• Each generated section is produced by a small helper returning a string.
• Output byte-for-byte equivalent (or better, tested for semantics).
• Tests pass.

Blocked by

None