feat: add posseg (part-of-speech tagging) HMM for OOV words

[messense]

Implement a compound-state HMM Viterbi (4 positions × 64 POS tags = 256 states) for POS-tagging unknown/OOV Chinese words, ported from Python jieba's posseg module.

• Add posseg.rs with dense 256×256 transition matrix for O(1) lookup
• Embed probability data (start, trans, emit, char_state_tab) via include_flate, gated on default-dict feature
• Improve tag() to use posseg HMM for OOV CJK words instead of falling back to "x" (e.g. "张尧" now correctly tagged as "nr"/person name)
• Add conversion script (scripts/convert_posseg.py) for regenerating posseg.txt from Python jieba's pickle files
• Add benchmark for tag_with_oov (~2.5µs per call)

[codspeed-hq]

Merging this PR will not alter performance

✅ 12 untouched benchmarks
🆕 1 new benchmark

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
🆕	tag_with_oov	N/A	78.2 µs	N/A

---

_{Comparing feat/posseg (18a9fff) with main (9e3965b)}

[codecov]

Codecov Report

❌ Patch coverage is 94.01993% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.85%. Comparing base (9e3965b) to head (18a9fff).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
jieba/src/posseg.rs	94.92%	14 Missing ⚠️
jieba/src/lib.rs	84.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   80.99%   82.85%   +1.85%     
==========================================
  Files           9       10       +1     
  Lines        1684     1971     +287     
==========================================
+ Hits         1364     1633     +269     
- Misses        320      338      +18     

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