refactor: decompose useEcharts into internal hooks

[chensid]

Summary

• Split monolithic useEcharts hook (404 → 148 LOC) into orchestrator + 6 focused internal hooks under src/hooks/internal/
• Add shallowEqual utility for option update deduplication, preventing unnecessary setOption calls
• Extract syncGroupConnectivity in connect.ts to eliminate duplicated connect/disconnect logic
• Fix clearInstanceCache to properly dispose all tracked instances before clearing
• Make EChart component responsive with height: 100% + minHeight: 400px (was fixed 400px)
• Add circular reference protection for theme objects in JSON.stringify
• Fix RAF ID not cleared after execution in resize observer

Test plan

• All 137 tests pass (vp test run)
• Zero lint/type errors (vp check)
• Build succeeds with correct outputs (vp pack)
• dist/index.d.ts does not expose any internal hooks
• Coverage: all internal hooks, utils, themes at 100%

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 97.19101% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.87%. Comparing base (d2dc5ef) to head (468fa6d).
⚠️ Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/hooks/use-echarts.ts	72.22%	5 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #345      +/-   ##
===========================================
- Coverage   100.00%   98.87%   -1.13%     
===========================================
  Files           10       18       +8     
  Lines          399      443      +44     
  Branches        92      104      +12     
===========================================
+ Hits           399      438      +39     
- Misses           0        5       +5     

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