Four FFI call-arg lowering sites resolved an argument's IR type via
`getRefIRType(arg_ref) orelse .void` — a silent fallback to the load-bearing
real type `.void`. A failed lookup there is a codegen invariant violation, but
`.void` is treated by downstream `toLLVMType` → `abiCoerceParamType` →
`coerceArg` as a legitimate void-typed foreign argument, corrupting the call
ABI with no diagnostic.
Add one shared resolver `LLVMEmitter.argIRTypeOrFail` that returns the
dedicated `.unresolved` sentinel on a failed lookup — never `.void`/`.s64` — so
the failure cannot masquerade as a real type and trips `toLLVMType`'s existing
hard `@panic` tripwire at the call site. Route all four sites through it:
- src/ir/emit_llvm.zig JNI constructor (NewObject) arg loop
- src/backend/llvm/ops.zig objc_msgSend arg loop
- src/backend/llvm/ops.zig JNI non-virtual call arg loop
- src/backend/llvm/ops.zig JNI Call<Type>Method arg loop
Happy path is byte-identical (every real arg already has a resolved type); FFI
examples stay green with zero snapshot churn.
Regression test (fail-before/pass-after) in src/ir/emit_llvm.test.zig asserts an
unresolvable FFI arg ref now yields `.unresolved`, not the old silent `.void`.
