Refactor god files – router & app split (follow-up)

[Tuntii]

Context

PR #199 split
outer.rs\ and \�pp.rs\ into submodules. This issue tracks follow-up hardening.

Follow-ups

• Further split \�pp/builder.rs\ (~1.6k lines) into routing / openapi / health / run modules
• Further split
  outer/tests.rs\ into unit vs property test files
• Add module-level docs in
  outer/mod.rs\ and \�pp/mod.rs\ for navigation
• Track file-size guard in CI (fail if any \src/**/*.rs\ > 50KB)

Acceptance

• No single
  ustapi-core/src/**/*.rs\ file exceeds 50KB
• All existing tests pass without behavior change

[Tuntii]

Primary refactor complete in PR (feat/issue-201-builder-split). builder.rs split into routing/openapi/health/run; 225 tests pass; API unchanged. Remaining follow-up: router/tests.rs split + optional CI file-size guard.

[Tuntii]

Closed via PR #202: builder.rs split into routing/openapi/health/run modules. All app/ files <50KB, 225 tests pass, no breaking changes.

[Tuntii]

Classifier round-4 fixes merged via PR #206. Builder split complete: routing/openapi/health/run modules, all \src/**/*.rs\ < 50KB, 226 lib tests pass, public API unchanged.