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).
