Fix segmentation for 习总书记

[yanyiwu]

Summary

• Add 习总书记 to the main dictionary so 和习总书记 no longer merges 和习 as one token.
• Add a MixSegment regression test for 马克思主义和习总书记新时代中国特色社会主义.

Notes

• This is a narrower alternative to Fix incorrect tokenization of 和习 in Xi-related phrases #224: it changes the main dictionary entry needed for the reported bad split without adding extra user-dictionary overrides for adjacent terms.
• Fixes 分词错误，‘和习’分成了一个词。 #223.

Verification

• make test passed locally on 2026-04-23.

Release assessment

• This is a user-visible dictionary correctness fix. A patch release should be considered after merge, but no cppjieba release has been published from this branch yet.