feat(asm): Phase E — multi-output asm returns tuples

Replaces the N>1 "Phase E" bail with a shared asmResultType helper (lowering +
inferType) that derives the result type from the out_value operands: 0→void,
1→T, N→a named tuple (fields named via the §II.5 effective-name rule).

Key realization: toLLVMType(tuple) already produces a literal struct {T1,…,Tn} —
exactly what LLVM's multi-output inline asm returns — so emit needs NO change.
Building the op with a tuple result type makes the asm call return the struct,
which IS sx's tuple value (destructured by the normal tuple_get path).

inferType's .asm_expr arm now also delegates to asmResultType (single owner), so
`return asm`, `x := asm`, and `q, r := asm` all agree on the type.

Verified end-to-end on aarch64: split(0x1234)→(lo=52,hi=18), a udiv/msub
divmod→(3,2). IR: `call { i64, i64 } asm "divq ${4}",
"={rax},={rdx},{rax},{rdx},r,~{cc}"(…)` → extractvalue → tuple.

1640 → the x86_64 multi-output IR lock (ir-only); 1647 → a multi-output example
that runs on aarch64.

zig build test green (655 corpus, 446 unit).

examples/1640-platform-asm-parse.sx

@@ -1,10 +1,10 @@
-// ASM stream — `asm { … }` parses + validates the full rich shape: named value
-// outputs (`[quot] "={rax}" -> u64`), register-pinned inputs, and a
-// `clobbers(.…)` clause, all accepted. This is a MULTI-output (tuple-returning)
-// asm, which is deferred to Phase E — so lowering bails LOUD + named with the
-// specific "Phase E" diagnostic (single-output asm already runs; see 1645).
-// Called from `main` so lowering reaches the asm body (lazy lowering skips
-// uncalled functions).
+// ASM stream Phase E — x86_64 multi-output asm: `divq` produces quotient in rax
+// and remainder in rdx, returned as a `(quot, rem)` tuple. Two `={rax}`/`={rdx}`
+// value outputs ⇒ LLVM returns a `{ i64, i64 }` struct, which IS sx's tuple
+// representation (so `q, r := …` destructures it directly). x86-pinned via
+// `.build`: ir-only on a non-x86 host (the `.ir` snapshot locks the struct
+// return + `%[name]` rewrite); runs natively on x86_64-linux. See 1647 for a
+// multi-output example that executes on aarch64.
 divmod :: (n: u64, d: u64) -> (quot: u64, rem: u64) {
     return asm {
         "divq %[d]",