Remove consumed frames in place

[utkarshalpha]

Closes #474.

FrameBuffer now stores incoming bytes in a bytearray, but consuming a frame still assigns self._data[9 + length:] back to the attribute. That creates and copies a new buffer for every frame, so parsing many buffered frames remains quadratic.

Delete the consumed prefix in place instead. CPython's optimized left deletion can then advance the bytearray start offset without copying the full remaining suffix. A regression test verifies both that the same bytearray object is retained and that the next frame remains buffered.

Benchmark

I buffered repeated valid, empty SETTINGS frames and then iterated the FrameBuffer on CPython 3.11 / Windows:

Frames	Before	After	Speedup
50,000	0.935s	0.157s	6.0x
100,000	4.018s	0.322s	12.5x
200,000	40.451s	0.641s	63.1x

The post-change throughput stays near 312,000 frames/s across the three input sizes.

Validation

• python -m pytest — 1,654 passed
• python -m mypy --strict-bytes src tests/typing/strict_bytes.py
• Ruff on the changed source and test (excluding three unrelated existing findings in test_basic_logic.py)
• git diff --check