docs(asm): note x86 %[fn:P] call modifier + checkpoint x86 coverage

docs/inline-assembly.md

@@ -87,6 +87,21 @@ Two reasons to prefer this over passing a function *pointer* in a plain
 - **Portable** — the backend emits the correctly-mangled name, so you
   don't hardcode the macOS leading underscore.
 
+On **x86_64**, a direct `call` to a symbol operand needs the `P`
+("call-target") operand modifier — `%[name:P]` (the GCC `%P0` idiom):
+
+```sx
+return asm volatile {
+    "call %[fn:P]",            // x86_64 — note the :P modifier
+    [ret] "={rax}" -> i64,
+    "{rdi}" = n,
+    [fn]   "s" = cb,
+    clobbers(.rcx, .rdx, .rsi, .r8, .r9, .r10, .r11, .memory),
+};
+```
+
+aarch64 `bl %[fn]` needs no modifier.
+
 The callee needs a stable, externally-linked symbol — i.e. `export`
 (which also gives it the C ABI). A plain or `callconv(.c)`-only function
 is `internal` and gets dead-code-eliminated, so the symbol won't link.