P2: Rename or restructure SSG to reflect Hono-specific implementation

[SisyphusZheng]

What to build

SSG currently imports hono/ssg and emits Hono-specific code. Either rename the package to ssg-hono or split the engine-agnostic core from the Hono driver.

Acceptance criteria

• Public docs and package name/structure match the actual implementation.
• Existing tests and gates pass.

Blocked by

None