* fix: remove debug console.log statements from ui.js
* fix: add error handling and rollback for temp directory cleanup
* fix: use streaming for hash calculation to reduce memory usage
* refactor: hoist CustomHandler require to top of installer.js and ui.js
* fix: fail fast on malformed custom module YAML
User customizations must be valid - silent skip hides broken configs.
* refactor: use consistent return type in handleMissingCustomSources
* refactor: clone config at install() entry to prevent mutation
