gnd: Extract shared manifest and watch modules to reduce duplication

Consolidate duplicated code across build and codegen commands:

- Create manifest.rs with unified Manifest types and load_manifest()
- Create watch.rs with generic watch_and_run() for --watch mode
- Consolidate compile_data_source_mapping/compile_template_mapping
- Consolidate copy_abi/copy_template_abi into copy_abi_to_dir
- Add output_extension() helper for wasm/wast conditionals

Removes ~225 lines of duplicated code while improving maintainability.

Author: lutter