Complete empty-struct edge coverage #100

Current state

Core empty structural behavior is implemented and accepted:

let marker: struct {} = .{};

Anonymous struct {} types and .{} values parse, typecheck, lower through HIR/MIR/LLVM, link, and run. Dedicated x_test/runtime_empty_struct coverage exercises locals, parameters, returns, aggregate fields, fixed arrays, and references. Named empty structs are also covered by x_test/runtime_interface_nested.

Empty structural types remain structural values, not nominal namespaces. Empty enum payload schemas remain distinct from payloadless enum cases.

Remaining scope

• Add explicit backend layout and ABI assertions for 32-bit and 64-bit targets.
• Verify constants, statics, owned pointers, dynamic arrays/slices, and address identity where applicable.
• Prove ownership and cleanup plans add no phantom drop work.
• Reject or explicitly define recursive anonymous by-value shapes before lowering.
• Preserve stable type text, fingerprints, interning, hover, and diagnostics while extending this matrix.

Validation already completed

• GOCACHE=/tmp/peeper-go-cache CCACHE_DISABLE=1 scripts/build.sh
• GOCACHE=/tmp/peeper-go-cache CCACHE_DISABLE=1 PEEPER_BIN="$PWD/build/bin/peeper" go test -count=1 ./x_test

This issue now tracks edge coverage only; it no longer claims core empty structs are unsupported.