[Layouts] Finish request-aware generated app and hybrid layout wiring

[cssbruno]

Source: split follow-up from #237 after the #61 routing/layout/view baseline.

Scope:

• Finish generated app wiring for request-aware layouts after the SSR/hybrid lifecycle contract is stable.
• Define and test hybrid layout behavior without changing the current explicit request-time lane selection rules.
• Keep package-scoped layout import semantics and qualified layout references intact.
• Ensure generated request-time layout composition uses runtime layout contracts instead of bypassing compiler IR.

Acceptance criteria:

• docs/language/layouts.md documents current request-aware and hybrid layout behavior without implying broader support than exists.
• Generated app tests cover request-aware layout composition when implemented.
• Hybrid layout behavior is either implemented with focused tests or intentionally deferred with clear diagnostics/docs.
• GOWDK product rules remain intact: full pages default to build-time output, and request-time rendering stays explicit through load {} or go ssr {}.

Related: #61, #237.