[cpunion]

Preserve native storage through definitions such as:

type Copy Native
p := (*Copy)(&frame.Value)
p.Tail = 99 // updates Value.Tail without corrupting the sentinel

Extend #2931's provenance foundation (original authored commit retained) with aggregate source layouts, LLVM-based reflected offsets/pointer prefixes, compatible typed-pointer/callback ABI checks, and versioned package representation metadata. Cover private origins, aliases, generic instances, HostLayout, SIMD storage, and memory32 native slots. Native arrays keep backing storage and indexed element slots consistent. Ordinary Go functions retain their calling convention.

Draft for the remaining #2927 migration: owner-aware source queries for native overrides, function-field address escapes, automatic array repacking between different layouts, and callback reflection/FFI still need work; see doc/design/native-representation.md. This does not close #2872 or all cases in #2873.

Validation: official Go, native LLGo O0/O3 full-LTO, and W32/J32/J64 execution pass. Full SSA/ABI/frontcache, CL -short, and internal/build -short (855s) suites pass with matching LLVM 22, plus target-layout, race, and cold/hot/cross-process cache checks.

[codecov]

Codecov Report

❌ Patch coverage is 55.44554% with 135 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
ssa/representation_conversion.go	21.42%	33 Missing ⚠️
internal/build/frontend_representation.go	0.00%	22 Missing ⚠️
ssa/representation.go	66.07%	19 Missing ⚠️
ssa/native_layout.go	62.85%	13 Missing ⚠️
ssa/representation_layout.go	62.85%	13 Missing ⚠️
ssa/type_layout.go	33.33%	8 Missing ⚠️
ssa/representation_metadata.go	63.15%	7 Missing ⚠️
internal/build/frontend_types.go	0.00%	6 Missing ⚠️
internal/build/frontend_link.go	0.00%	4 Missing ⚠️
internal/build/frontend_prepare.go	0.00%	3 Missing ⚠️
... and 5 more

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