refactor: drop @jridgewell/trace-mapping from source-map composition

Replace `TraceMap`/`decodedMappings`/`originalPositionFor` with an
in-house Base64-VLQ decoder + segment lookup in `src/minify.js`. The
composer now reads, looks up, and re-emits source maps using only
plain JavaScript — no external dependency.

Also handle minimizers that return the map as a JSON string
(terser's default) so that user-provided `minify` functions which
forward `terser.minify(...)` keep working through the dispatcher's
chaining step.

Author: claude