Add onboarding comments

[mrecachinas]

Summary

• Add onboarding comments to the Rust dispatch layer, lookup tables, and SIMD fallback paths.
• Add test comments explaining API groups, threshold checks, buffer-protocol coverage, and benchmark intent.
• Add Criterion benchmark comments explaining input shapes and measurement goals.

Testing

• cargo fmt --check
• ruff check .
• ruff format --check .
• git diff --check
• Documented smoke validation snippets
• python3 -m pytest -vls . (2 existing Python 3.14/PyO3 TypeError wording expectation failures)

[github-actions]

📊 Benchmark Comparison Results

Status	Benchmark	Base	PR	Delta
❌	check_bytes_arrays_first_within_dist[16384 elems,s=64,end]	0.0930ms	0.1331ms	43.2% slower
❌	check_bytes_arrays_first_within_dist[16384 elems,s=64,mid]	0.0463ms	0.0661ms	42.8% slower

➖ 43 benchmarks within noise (click to expand)

Benchmark	Base	PR	Delta
check_bytes_arrays_all_within_dist[ 512 elems,s=16,at 0]	0.0039ms	0.0037ms	4.5% faster
check_bytes_arrays_all_within_dist[ 512 elems,s=16,end]	0.0039ms	0.0037ms	4.9% faster
check_bytes_arrays_all_within_dist[ 512 elems,s=16,mid]	0.0039ms	0.0037ms	4.8% faster
check_bytes_arrays_all_within_dist[ 1024 elems,s=32,at 0]	0.0084ms	0.0081ms	3.7% faster
check_bytes_arrays_all_within_dist[ 1024 elems,s=32,end]	0.0084ms	0.0081ms	3.8% faster
check_bytes_arrays_all_within_dist[ 1024 elems,s=32,mid]	0.0085ms	0.0081ms	4.1% faster
check_bytes_arrays_all_within_dist[16384 elems,s=64,at 0]	0.0763ms	0.0858ms	12.4% slower
check_bytes_arrays_all_within_dist[16384 elems,s=64,end]	0.0774ms	0.0871ms	12.5% slower
check_bytes_arrays_all_within_dist[16384 elems,s=64,mid]	0.0763ms	0.0859ms	12.6% slower
check_bytes_arrays_best_within_dist[ 512 elems,s=16,at 0]	0.0048ms	0.0046ms	4.0% faster
check_bytes_arrays_best_within_dist[ 512 elems,s=16,end]	0.0043ms	0.0043ms	0.6% slower
check_bytes_arrays_best_within_dist[ 512 elems,s=16,mid]	0.0046ms	0.0045ms	0.9% faster
check_bytes_arrays_best_within_dist[ 1024 elems,s=32,at 0]	0.0092ms	0.0093ms	0.2% slower
check_bytes_arrays_best_within_dist[ 1024 elems,s=32,end]	0.0093ms	0.0093ms	0.1% slower
check_bytes_arrays_best_within_dist[ 1024 elems,s=32,mid]	0.0093ms	0.0093ms	0.0% faster
check_bytes_arrays_best_within_dist[16384 elems,s=64,at 0]	0.0936ms	0.0980ms	4.7% slower
check_bytes_arrays_best_within_dist[16384 elems,s=64,end]	0.0935ms	0.0974ms	4.2% slower
check_bytes_arrays_best_within_dist[16384 elems,s=64,mid]	0.0934ms	0.0981ms	5.0% slower
check_bytes_arrays_first_within_dist[ 512 elems,s=16,at 0]	0.0002ms	0.0002ms	13.6% faster
check_bytes_arrays_first_within_dist[ 512 elems,s=16,end]	0.0042ms	0.0039ms	7.3% faster
check_bytes_arrays_first_within_dist[ 512 elems,s=16,mid]	0.0023ms	0.0021ms	7.6% faster
check_bytes_arrays_first_within_dist[ 1024 elems,s=32,at 0]	0.0002ms	0.0002ms	14.2% faster
check_bytes_arrays_first_within_dist[ 1024 elems,s=32,end]	0.0089ms	0.0086ms	4.2% faster
check_bytes_arrays_first_within_dist[ 1024 elems,s=32,mid]	0.0047ms	0.0044ms	4.7% faster
check_bytes_arrays_first_within_dist[16384 elems,s=64,at 0]	0.0002ms	0.0002ms	11.7% faster
check_bytes_within_dist[127 bytes,d=500]	0.0002ms	0.0003ms	25.8% slower
check_bytes_within_dist[16 bytes,d=0]	0.0002ms	0.0002ms	21.3% faster
check_bytes_within_dist[64 bytes,d=100]	0.0002ms	0.0002ms	22.1% faster
hex_within_dist	0.0001ms	0.0001ms	5.0% slower
hd_bytes[1000-diff]	0.0002ms	0.0002ms	0.7% faster
hd_bytes[1000-same]	0.0001ms	0.0001ms	0.5% slower
hd_bytes[1024-diff]	0.0002ms	0.0001ms	4.9% faster
hd_bytes[1024-same]	0.0001ms	0.0001ms	5.1% faster
hd_bytes[3-diff]	0.0001ms	0.0001ms	5.6% faster
hd_bytes[3-same]	0.0001ms	0.0001ms	5.9% faster
hd_bytes[64-diff]	0.0001ms	0.0001ms	4.9% faster
hd_string[1000-diff]	0.0001ms	0.0002ms	44.4% slower
hd_string[1000-same]	0.0001ms	0.0002ms	53.5% slower
hd_string[1024-diff]	0.0001ms	0.0002ms	47.0% slower
hd_string[1024-same]	0.0001ms	0.0002ms	55.0% slower
hd_string[3-diff]	0.0002ms	0.0002ms	6.4% faster
hd_string[3-same]	0.0001ms	0.0001ms	16.4% slower
hd_string[64-diff]	0.0001ms	0.0001ms	7.2% slower

Legend: ✅ Faster (>5%) · ⚠️ Slower (5-30%) · ❌ Regression (>30%) · 🆕 New · 🗑️ Removed

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⛔ This PR has severe performance regressions (>20% slower). Please investigate before merging.