refactor: use @jridgewell/trace-mapping for source-map composition

Read maps with `TraceMap`/`decodedMappings`/`originalPositionFor`/
`sourceContentFor` from `@jridgewell/trace-mapping` (already a direct
dep) instead of an in-house decoder, and keep the small inline VLQ
encoder for emitting the composed `mappings` string.

Author: claude