Add a separate BundleBuilder executable that packages a module graph to .gcbundle

Goal

Ship packaging as a dedicated BundleBuilder executable instead of overloading ScriptLoader.

Scope

• Add a new BundleBuilder CLI.
• Resolve the entry module graph.
• Compile each script module to bytecode.
• Embed JSON modules.
• Write .gcbundle output.
• Reject unsupported references such as unresolved imports, dynamic import(), and paths outside the configured bundle root.

Acceptance Criteria

• BundleBuilder ./app.js --output app.gcbundle produces a runnable bundle.
• Unresolved imports and unsupported references hard-fail with explicit errors.
• The tool is documented in the build/run docs.
• CLI tests cover success and failure cases.

Stand-alone Value

Provides a clean build-time packaging tool even before native executable packaging exists.

Dependencies

Depends on #153, #154, and #155.

Tracking

Tracked by #161.

Start by reviewing ScriptLoader and the work in dependencies #153, #154, and #155, then trace the app.js entry point and existing CLI conventions. Use the build/run docs and CLI test area as the validation targets; done means a runnable .gcbundle is produced and unsupported references fail with explicit errors.