feat(asm): Phase A.1 — parse asm { … } into AsmExpr; loud lowering bail

`asm volatile? { "tmpl", [name]? "constraint" (-> Type | = expr), …,
clobbers(.…) }` now parses into a flat-operand AsmExpr/AsmOperand (ast.zig +
parser.zig parseAsmExpr, dispatched from parsePrimary on .kw_asm). `volatile`
and `clobbers` are recognized contextually (not reserved). `-> @place`
write-through is rejected with a clear "Phase 2" parse error.

Codegen is not implemented yet (IR op + LLVM emit are Phases C–E), so lowering
bails LOUD + named via an explicit .asm_expr arm in lower/expr.zig (not the
generic unknown_expr else) — emitPlaceholder makes hasErrors() abort the build
on the message.

The new asm_expr tag forced (and got) arms in three exhaustive Node.Data
switches: sema.zig analyzeNode + findNodeAtOffset, semantic_diagnostics.zig
checkBindingNames — each recurses into template + operand payloads.

Design: adopted the operand auto-naming rule (design §II.5) — name auto-derived
from a {reg} pin, explicit [name] only when it differs or for register-class
operands, echo form rejected. Typing-stage rule; parser stores name: ?[]const u8.

Locked with examples/1640-platform-asm-parse.sx (multi-output divmod: named
operands, register pins, clobbers — parses then bails, called from main).

Also files issue 0137 (pre-existing, orthogonal: `sx run` with no `main`
segfaults via an unguarded JIT entry lookup in target.zig — not an asm bug).

zig build test green (648 corpus, 445 unit).

examples/expected/1640-platform-asm-parse.exit

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

examples/expected/1640-platform-asm-parse.stderr

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+error: inline assembly codegen is not yet implemented (ASM stream: lowering + emit land in Phases C–E)
+  --> examples/1640-platform-asm-parse.sx:9:12
+   |
+ 9 |     return asm {
+   |            ^^^^^
+10 |         "divq %[d]",
+   | ^^^^^^^^^^^^^^^^^^^^
+11 |         [quot] "={rax}" -> u64,
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+12 |         [rem]  "={rdx}" -> u64,
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+13 |         "{rax}" = n,  "{rdx}" = 0,  [d] "r" = d,
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+14 |         clobbers(.cc),
+   | ^^^^^^^^^^^^^^^^^^^^^^
+15 |     };
+   | ^^^^^

examples/expected/1640-platform-asm-parse.stdout

@@ -0,0 +1 @@
+