perf(editor): reduce frequency of native selection sync

[doodlewind]

Currently, the native selection sync logic is costly in large docs. While there is already a throttle(10ms) in place, the actual DOM querying cost can take several hundred milliseconds, leading to multiple redundant executions queuing up and high CPU usage.

This PR implements a custom serialThrottle that ensures only one execution runs at a time and processes only the latest selection state once the previous execution completes. This eliminates redundant DOM queries without affecting the selection sync accuracy, effectively reducing CPU usage while maintaining UI responsiveness.

Tested using a markdown novel with ~20k words (100k characters).

moby-dick-chap1-10.md

Before:

After:

[graphite-app]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 52.89%. Comparing base (cfd64f1) to head (e0e3db7).
Report is 3 commits behind head on canary.

Additional details and impacted files

@@            Coverage Diff             @@
##           canary    #9509      +/-   ##
==========================================
- Coverage   52.91%   52.89%   -0.02%     
==========================================
  Files        2152     2151       -1     
  Lines       97039    97039              
  Branches    16161    16162       +1     
==========================================
- Hits        51350    51331      -19     
- Misses      44322    44340      +18     
- Partials     1367     1368       +1     

Flag	Coverage Δ
server-test	78.07% <ø> (-0.07%)	⬇️
unittest	32.51% <100.00%> (+<0.01%)	⬆️

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