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/1644-platform-asm-duplicate-name.sx

@@ -0,0 +1,4 @@
+// ASM stream Phase B — two asm operands may not share a `[name]`: the `%[name]`
+// template reference (and the result tuple field) would be ambiguous.
+f :: () -> u64 { return asm volatile { "nop", [x] "=r" -> u64, [x] "r" = 5 }; }
+main :: () { v := f(); }