feat(bundler): Integrate rolldown bundler support and update bundler selection logic

readme.hints.md

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+# tsbundle Hints and Findings
+
+## Bundler Architecture
+- tsbundle uses a child process architecture where each bundler runs in a separate process
+- Configuration is passed via environment variables as JSON (IEnvTransportOptions)
+- The main class `TsBundle` spawns child processes using `smartspawn.ThreadSimple`
+
+## Bundler Implementations
+- **esbuild** (default): Fully implemented, production ready
+- **rolldown**: Implemented and working (beta), produces smaller bundles than esbuild
+- **rollup**: Empty module directory exists but not implemented
+- **parcel**: Empty module directory exists but not implemented
+
+## Adding New Bundlers
+To add a new bundler, you need:
+1. Update `ICliOptions` interface to include the bundler name
+2. Add case in `getBundlerPath()` switch statement
+3. Create `mod_<bundler>/` directory with:
+   - `plugins.ts`: Import and re-export the bundler
+   - `index.child.ts`: Implement TsBundleProcess class with buildTest() and buildProduction()
+4. Add bundler to package.json dependencies
+
+## Rolldown Specific Notes
+- Rolldown is in beta (v1.0.0-beta.18) but working well
+- API: Use `rolldown()` function directly, not `rolldown.rolldown()`
+- Output options go in the `write()` method, not the initial config
+- Produces smaller minified bundles than esbuild (1.5K vs 2.2K in tests)
+- Supports TypeScript via `resolve.tsconfigFilename`
+
+## CLI Usage
+- Default bundler: `tsbundle`
+- Specify bundler: `tsbundle --bundler=rolldown`
+- Production mode: `tsbundle --production`
+- Combined: `tsbundle --bundler=rolldown --production`
+
+## Known Issues
+- esbuild recently had splitting and tree-shaking disabled due to issues
+- The README still mentions "a bundler using rollup" but uses esbuild by default