v5: split into PbfReader and PbfWriter

[mourner]

Breaking change: the single Pbf class is split into PbfReader and PbfWriter, allowing readers and writers to be tree-shaken independently in bundled apps.

Removed: the legacy readVarint64 method (replaced by readVarint(true) for signed 64-bit reads).

Performance: several targeted optimizations to both read and write hot paths, validated against bench/bench-tiles.js:

• makeRoomForExtraLength uses Uint8Array.copyWithin instead of a manual byte-shift loop (biggest write win).
• writeVarint gets a single-byte fast path, skipping the multi-byte branches and full realloc(4) for the common case (tags, small ints).
• readVarint gets a matching single-byte fast path.
• writeBytes uses typedArray.set instead of a manual loop.

End-to-end on the tile benchmark: write throughput improved from ~169 MB/s to ~215 MB/s (~27% faster), and read throughput from ~150 MB/s to ~157 MB/s (~5% faster).