feat: inspect generic structs (approximate layout + disclaimer)

[peczenyj]

Inspect mode used to silently skip generic types. Now it shows a best-effort
layout with a clear disclaimer.

Behavior

$ structalign -inspect -type=Generic ./_example
// generic type — layout assumes T=any; the real layout depends on the type argument(s)
type Generic[T] struct { // size: 32, align: 8, padding: 11
	Flag bool    // size:  1, align: 1, padding: 7
	Value any    // size: 16, align: 8
	Count uint32 // size:  4, align: 4, padding: 4
}

Each type parameter is replaced by a representative type — its constraint's core
type (~int → int), or interface{} when unbounded (any/comparable/unions).
The disclaimer line + the [T] in the header make the approximation explicit.

Changes

• common.Layout gains TypeParams + Note; ui.renderLayout renders them.
• layout.resolveStruct/representativeType instantiate via types.Instantiate
  (validate=false); non-struct / non-instantiable types are still skipped.
• README "Inspecting generic types" explainer (why it's hard + the disclaimer);
  AGENTS.md and the generic caveat updated.

task ci green; new layout/ui tests cover the any/core-type/non-struct paths.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 82.72727% with 19 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.83%. Comparing base (4ca09d1) to head (b676874).

Files with missing lines	Patch %	Lines
internal/layout/layout.go	77.10%	16 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##            devel      #17      +/-   ##
==========================================
+ Coverage   83.07%   83.83%   +0.76%     
==========================================
  Files          10       10              
  Lines         573      662      +89     
==========================================
+ Hits          476      555      +79     
- Misses         56       67      +11     
+ Partials       41       40       -1     

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