feat(asm): Phase B.1 — operand-name validation (echo + duplicates)

Extends lowerAsmExpr with a pinnedRegister(constraint) helper and two §II.5
operand-naming checks, in the compile path before the codegen bail:

- reject the echo form `[eax] "={eax}"` — a label identical to the register its
  own constraint pins is redundant (the operand is already auto-named after the
  register); the useful form is a label that differs (`[quot] "={rax}"`);
- reject duplicate operand names (ambiguous %[name] / result field).

Locked with 1643-platform-asm-echo-name and 1644-platform-asm-duplicate-name.

zig build test green (652 corpus, 445 unit).

examples/1643-platform-asm-echo-name.sx

@@ -0,0 +1,6 @@
+// ASM stream Phase B — operand naming rule (§II.5): an explicit `[name]` that
+// just echoes the register its own constraint pins (`[eax] "={eax}"`) carries
+// no information — the operand is already auto-named after the register. Reject
+// it. The useful form is a label that DIFFERS (e.g. `[quot] "={rax}"`).
+f :: () -> u32 { return asm volatile { "cpuid", [eax] "={eax}" -> u32, "{eax}" = 1 }; }
+main :: () { x := f(); }