Document CompactString rationale in EnhancedCell module

[ryanoneill]

Summary

• Adds module-level documentation explaining why CompactString is used for cell symbols
• At 80×24, a terminal has 1,920 cells per frame; CompactString stores up to 24 bytes inline, eliminating thousands of small heap allocations per render cycle

Test plan

• Documentation-only change, no behavioral impact
• cargo doc --all-features builds cleanly

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.09%. Comparing base (fa6edbb) to head (8d606dc).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #222   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files         146      146           
  Lines       12798    12798           
=======================================
  Hits        11786    11786           
  Misses       1012     1012           

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