Add compound-components feature flag

[ryanoneill]

Summary

• Adds a new compound-components feature flag that gates all 8 compound components: Form, SearchableList, SplitPanel, DataGrid, LogViewer, ChatView, MetricsDashboard, Chart
• The feature transitively enables input-components, data-components, and display-components (the dependencies compound components need)
• Added to the full feature, so default behavior is unchanged
• Users who only need basic component groups can now skip compiling compound components

Test plan

• cargo check --all-features passes
• cargo check --no-default-features compiles (compound components excluded)
• cargo check --no-default-features --features input-components compiles clean
• cargo test --all-features passes (2882 unit + 267 doc)
• cargo clippy --all-features -- -D warnings clean

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.82%. Comparing base (ff80ab6) to head (f6663f0).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #104   +/-   ##
=======================================
  Coverage   92.82%   92.82%           
=======================================
  Files          94       94           
  Lines        9471     9471           
=======================================
  Hits         8791     8791           
  Misses        680      680           

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