Perf: split input once with mb_str_split in the main parse loop

[mmucklo]

Realizes the ROADMAP main-loop performance item — and the new benchmark CI job (#59) measures it automatically.

Change

The main parse loop called mb_substr($emails, $i, 1, $encoding) for every character, plus once per character for each comment/quoted look-ahead. For multi-byte encodings mb_substr rescans from the start of the string to locate the Nth character — O(n) per call, O(n²) across the loop. This splits the input once with mb_str_split() and indexes the resulting array: a single O(n) pass.

The diff is confined to the main loop — the three mb_substr($emails, …) sites become $chars[$i] / $chars[$j].

Results (composer bench:compare vs v3.3.2 baseline, same host)

Subject	Before	After	Δ
Simple ASCII	103.7 μs	86 μs	−17%
Name-addr	137.3 μs	104 μs	−24%
UTF-8 local part	130.4 μs	98 μs	−25%
Obs-route	108.7 μs	81 μs	−26%
Comment extraction	152.5 μs	114 μs	−25%
Batch 10	682 μs	541 μs	−21%
Batch 100 (stream)	13,415 μs	9,783 μs	−27%

Consistent 10–27% across the suite; biggest gains on longer inputs, as expected for an O(n²)→O(n) fix.

Safety

• Behavior-preserving: all 91 tests pass unchanged, PHPStan level 8 / Psalm / cs clean.
• mb_str_split() is encoding-aware (same $encoding), so multi-byte handling is identical — verified by the existing UTF-8/ISO-8859-1/Shift-JIS tests.
• $emails/$len are not mutated inside the loop, so pre-splitting is safe.

One trade-off worth noting: the character array costs memory proportional to input length (bounded by input size). For very large single batches a chunked reader would cap that — noted as a ROADMAP follow-up.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.07%. Comparing base (f109878) to head (30a664d).

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #60      +/-   ##
============================================
- Coverage     93.17%   93.07%   -0.10%     
  Complexity      380      380              
============================================
  Files             6        6              
  Lines           981      982       +1     
============================================
  Hits            914      914              
- Misses           67       68       +1     

Files with missing lines	Coverage Δ
src/Parse.php	90.36% <100.00%> (-0.14%)	⬇️

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