refactor(codegen): remove Buffer dependency from source maps (#25914)

## Summary

- replace Node.js `Buffer` storage with `Uint8Array` in the source-map encoder
- use `TextDecoder` to create the final mappings string
- use `Uint8Array#set` when growing the mappings buffer
- remove the `node:buffer` runtime dependency

This makes source-map generation runtime-neutral and removes a blocker for browser support for the codegen package.

## Performance

Indexed access, copying, and ASCII decoding have comparable performance. `Uint8Array` allocation requires zero-initialization, unlike `Buffer.allocUnsafe`, but the expected end-to-end impact is small.