ERR/E5.2: comptime #run of an escaping failable → diagnostic + halt

A bare failable `#run` (no catch/or) whose error escapes used to segfault (const
form `x :: #run f()`) or silently succeed (statement form `#run f();`). Now the
compiler reports the raised tag name + the resolved return trace at the #run site
and halts with a non-zero exit.

- lower.zig: a failable #run's comptime function returns the full failable tuple
  (so the error slot is inspectable) while the global is typed as the success
  value; failable side-effects return the tuple instead of void.
- emit_llvm.zig: read the always-on comptime trace buffer (extern sx_trace_*);
  comptimeErrChannel + checkComptimeFailable split the result (non-zero tag →
  reportComptimeEscape + comptime_failed flag; success → value part). Wired into
  emitGlobals (const) and runComptimeSideEffects (statement, now filtered by the
  __run name; buffer cleared before each eval).
- core.zig: generateCode returns error.ComptimeError when comptime_failed, so the
  driver aborts before JIT/link.

catch / or / onfail compose at comptime exactly as at runtime; a successful bare
#run yields the value. Regressions: examples/1037-errors-comptime-run-escape
(diagnostic, exit 1) + 1038-errors-comptime-run-handled (exit 164). Suite: 326.

src/core.zig

@@ -152,6 +152,9 @@ pub const Compilation = struct {
         // callbacks can re-enter the interpreter via `invokeByName`.
         self.ir_module = ir_mod_ptr;
         self.ir_emitter = emitter;
+        // A comptime `#run` raised an unhandled error — the diagnostic + trace
+        // were already printed to stderr; abort before JIT/link (E5.2).
+        if (emitter.comptime_failed) return error.ComptimeError;
     }
 
     /// Re-enter the IR interpreter after `generateCode` (and after linking,