👌 Make StateBase.src a plain attribute (~6% faster parsing)

[deadstrobe5]

StateBase.src was a property whose setter only reset the cache of the deprecated srcCharCode. Rules read state.src constantly (~120k reads per render of benchmarking/samples/spec.md), so the property call alone was a measurable share of parse time.

This makes src a plain attribute. The srcCharCode cache is now a class-level (src, codes) pair, rebuilt whenever src is not the string it was built from. It is class-level because StateBlock, StateInline and StateCore don't call StateBase.__init__.

Benchmark

pytest benchmarking/bench_core.py, Apple M5, Python 3.11, 3 runs each side:

	Before	After
median	62.0–63.4 ms	57.7–60.7 ms
min	58.8–60.1 ms	55.1–55.9 ms

Interleaved A/B, 15 processes per side: median 61.1 → 57.0 ms (-6.7%), faster in 14 of 15 pairs.

Safety

• Output is byte-identical (HTML and token dicts) across all test fixtures and benchmark samples for the commonmark, default, gfm-like, zero and js-default presets.
• Full test suite passes (1033), plus the mdit-py-plugins suite (511). ruff and mypy --strict at the pre-commit pinned versions are clean.
• Added a test that srcCharCode follows reassignment of src.

Independent of #449 (different file); the two should stack.

Thanks for maintaining markdown-it-py!

Re-measured on CPython 3.14.7 (7 alternating processes per side, median): rendering benchmarking/samples/spec.md went from 26.6 ms to 25.3 ms (about 5%).