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/expected/1643-platform-asm-echo-name.exit

@@ -0,0 +1 @@
+1

examples/expected/1643-platform-asm-echo-name.stderr

@@ -0,0 +1,5 @@
+error: redundant asm operand name `eax` — it already names the pinned register; drop the `[eax]`
+  --> examples/1643-platform-asm-echo-name.sx:5:25
+   |
+ 5 | f :: () -> u32 { return asm volatile { "cpuid", [eax] "={eax}" -> u32, "{eax}" = 1 }; }
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1643-platform-asm-echo-name.stdout

@@ -0,0 +1 @@
+

examples/expected/1644-platform-asm-duplicate-name.exit

@@ -0,0 +1 @@
+1

examples/expected/1644-platform-asm-duplicate-name.stderr

@@ -0,0 +1,5 @@
+error: duplicate asm operand name `x`
+  --> examples/1644-platform-asm-duplicate-name.sx:3:25
+   |
+ 3 | f :: () -> u64 { return asm volatile { "nop", [x] "=r" -> u64, [x] "r" = 5 }; }
+   |                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

examples/expected/1644-platform-asm-duplicate-name.stdout

@@ -0,0 +1 @@
+