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

current/CHECKPOINT-ASM.md

@@ -368,6 +368,13 @@ Orthogonal: **issue 0137** (no-`main` segfault).
   locked the rejection (replacing an LLVM-verifier crash), then implemented +
   flipped to a runnable aarch64 example (objdump-confirmed direct `bl <_cb>`).
   `zig build test` green (665 corpus, 446 unit).
+- (x86 cross-arch) ir-only x86_64 siblings so each emit path is locked on BOTH
+  arches: 1657 read-write (`"incq ${0}","=r,0"`), 1658 indirect (`"movq $$42,
+  ${0}","=*m"`(ptr elementtype)), 1659 symbol (`"call ${2:P}"`, direct call). x86
+  templates validated by cross-emitting an object (integrated assembler accepts;
+  objdump confirms 1659's direct `call` reloc). **Note:** x86 direct calls need
+  the `P` operand modifier (`%[fn:P]`); aarch64 `bl %[fn]` needs none. Pure
+  additive locks. `zig build test` green (668 corpus, 446 unit).
 
 ## Known issues
 - **0138** — RESOLVED. `@const` (address-of a `::` comptime constant) yielded a