feat: improve snapping to grid in selector middleware

[chenshenhai]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 105 lines in your changes missing coverage. Please review.

Project coverage is 6.47%. Comparing base (ee87f3c) to head (9ea6699).
Report is 125 commits behind head on main.

Files	Patch %	Lines
packages/util/src/lib/flat.ts	0.00%	48 Missing and 24 partials ⚠️
packages/board/src/lib/calculator.ts	0.00%	15 Missing and 4 partials ⚠️
packages/core/src/middleware/selector/index.ts	0.00%	9 Missing and 4 partials ⚠️
packages/idraw/src/idraw.ts	0.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@           Coverage Diff            @@
##            main    #327      +/-   ##
========================================
- Coverage   6.54%   6.47%   -0.08%     
========================================
  Files        105     106       +1     
  Lines       6743    6816      +73     
  Branches    1315    1346      +31     
========================================
  Hits         441     441              
- Misses      5042    5084      +42     
- Partials    1260    1291      +31     

Flag	Coverage Δ
unittests	6.47% <0.00%> (-0.08%)	⬇️

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