Add DebugReplaceableView and CustomViewCountCache API

[Kyle-Ye]

Summary

• Add DebugReplaceableView - a type that erases view opaque result types in debug builds, enabling Xcode Previews to replace view definitions without full rebuilds
• Add CustomViewCountCache - a cache for tracking custom view counts during view list enumeration
• Add DebugReplaceableViewInfo for debug replaceable view count tracking
• Update _ViewListCountInputs with customViewCache and debugReplaceableViewInfo fields for 2025 API
• Update CustomViewModifier to support 2025 API view list counting

Test plan

• Build succeeds with swift build
• Existing tests pass with swift test

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 13.61%. Comparing base (9afbb33) to head (9f88ddc).
⚠️ Report is 2 commits behind head on main.

❗ There is a different number of reports uploaded between BASE (9afbb33) and HEAD (9f88ddc). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (9afbb33)	HEAD (9f88ddc)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #747       +/-   ##
===========================================
- Coverage   27.23%   13.61%   -13.62%     
===========================================
  Files         626      598       -28     
  Lines       38201    35300     -2901     
===========================================
- Hits        10403     4806     -5597     
- Misses      27798    30494     +2696     

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