feat(asm): symbol operands ("s") — direct call/branch to a function

A `"s"` input operand feeds a function/global symbol; the template's
%[name] emits the platform-mangled name, so `bl %[fn]` / `call %[fn]`
branches DIRECTLY to it (PC-relative, no register load — one fewer
indirection than register-indirect `blr`).

Lowering: an `"s"` input lowers its RHS normally (a function name →
`ptr @fn`); the rejection added last commit is removed. Emit: a symbol
operand is passed with its OWN llvm type (LLVMTypeOf) and no coercion —
the function value is a `ptr`, and the old coerce-to-register-int path
mistyped it and failed the verifier. New asmIsSymbol helper.

Verified on aarch64: examples/1656 (sx → asm → bl _cb → sx → 42); the
emitted asm is a direct `bl <_cb>` (objdump-confirmed), IR constraint
`...,s,...`(ptr @cb). Flipped 1656 from the rejection lock to a runnable
aarch64 example. zig build test green (665 corpus, 446 unit).

examples/expected/1656-platform-asm-symbol-operand.build

@@ -0,0 +1 @@
+{ "target": "macos" }

examples/expected/1656-platform-asm-symbol-operand.exit

@@ -1 +1 @@
-1
+42

examples/expected/1656-platform-asm-symbol-operand.ir

@@ -0,0 +1,28 @@
+
+; Function Attrs: nounwind
+define i64 @cb(i64 %0) #0 {
+entry:
+  %alloca = alloca i64, align 8
+  store i64 %0, ptr %alloca, align 8
+  %load = load i64, ptr %alloca, align 8
+  %add = add i64 %load, 1
+  ret i64 %add
+}
+
+; Function Attrs: nounwind
+define internal i64 @tramp(i64 %0) #0 {
+entry:
+  %alloca = alloca i64, align 8
+  store i64 %0, ptr %alloca, align 8
+  %load = load i64, ptr %alloca, align 8
+  %asm = call i64 asm sideeffect "        stp x29, x30, [sp, #-16]!\0A        mov x0, ${1}\0A        bl ${2}\0A        mov ${0}, x0\0A        ldp x29, x30, [sp], #16\0A", "=r,r,s,~{x0},~{x30},~{memory}"(i64 %load, ptr @cb)
+  ret i64 %asm
+}
+
+; Function Attrs: nounwind
+define i32 @main() #0 {
+entry:
+  %call = call i64 @tramp(i64 41)
+  %ca.tr = trunc i64 %call to i32
+  ret i32 %ca.tr
+}

examples/expected/1656-platform-asm-symbol-operand.stderr

@@ -1,29 +1 @@
-error: symbol asm operands (`"s"`) are not yet implemented
-  --> examples/1656-platform-asm-symbol-operand.sx:10:12
-   |
-10 |     return asm volatile {
-   |            ^^^^^^^^^^^^^^
-11 |         #string ASM
-   | ^^^^^^^^^^^^^^^^^^^
-12 |         stp x29, x30, [sp, #-16]!
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-13 |         mov x0, %[arg]
-   | ^^^^^^^^^^^^^^^^^^^^^^
-14 |         bl %[fn]
-   | ^^^^^^^^^^^^^^^^
-15 |         mov %[res], x0
-   | ^^^^^^^^^^^^^^^^^^^^^^
-16 |         ldp x29, x30, [sp], #16
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-17 | ASM,
-   | ^^^^
-18 |         [res] "=r" -> i64,
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
-19 |         [arg] "r" = n,
-   | ^^^^^^^^^^^^^^^^^^^^^^
-20 |         [fn]  "s" = cb,             // symbol operand → direct `bl _cb`
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-21 |         clobbers(.x0, .x30, .memory),
-   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-22 |     };
-   | ^^^^^
+