fix(installer): support local custom-source modules in resolveInstalledModuleYaml and TOML key (#2316)

- resolveInstalledModuleYaml: fall back to CustomModuleManager._resolutionCache for local
  custom-source modules (external cache path doesn't exist for these); refactor candidate-path
  search into shared searchRoot() helper; add *-setup/assets/module.yaml BMB standard path
- manifest-generator: use module code field (not display name) as TOML section key [modules.X]

Co-authored-by: cidemaxio <cidemaxio@users.noreply.github.com>