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.

examples/1037-errors-comptime-run-escape.sx

@@ -0,0 +1,18 @@
+// Comptime `#run` of a failable whose error ESCAPES (no `catch` / `or`): the
+// compiler reports the raised tag name + the return trace at the `#run` site and
+// halts with a non-zero exit (E5.2). Before this, a bare failable `#run`
+// segfaulted (const form) or silently succeeded (statement form).
+
+#import "modules/std.sx";
+
+E :: error { Bad, Empty }
+
+parse :: (n: s32) -> (s32, !E) {
+    if n < 0  { raise error.Bad; }
+    if n == 0 { raise error.Empty; }
+    return n * 2;
+}
+
+x :: #run parse(-1);            // error.Bad escapes → comptime diagnostic + halt
+
+main :: () -> s32 { return x; }

examples/1038-errors-comptime-run-handled.sx

@@ -0,0 +1,31 @@
+// Comptime `#run` of a failable composes with the handlers exactly as at
+// runtime: `catch` absorbs, `or` terminates, a successful bare `#run` yields the
+// value (error channel stripped), and an `onfail` in the evaluated body still
+// runs during comptime unwinding (E5.2).
+
+#import "modules/std.sx";
+
+E :: error { Bad, Empty }
+
+parse :: (n: s32) -> (s32, !E) {
+    if n < 0  { raise error.Bad; }
+    if n == 0 { raise error.Empty; }
+    return n * 2;
+}
+
+guard :: (ok: bool) -> !E {
+    onfail print("comptime cleanup\n");
+    if !ok { raise error.Bad; }
+    return;
+}
+
+ok_v   :: #run parse(5);            // success → 10 (value, error stripped)
+caught :: #run parse(-1) catch e 99; // Bad   → 99
+ored   :: #run parse(0) or 55;       // Empty → 55
+
+#run guard(false) catch e { };       // onfail fires during the comptime unwind
+
+main :: () -> s32 {
+    print("ok={} caught={} ored={}\n", ok_v, caught, ored);
+    return ok_v + caught + ored;     // 10 + 99 + 55 = 164
+}

examples/expected/1037-errors-comptime-run-escape.exit

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

examples/expected/1037-errors-comptime-run-escape.stderr

@@ -0,0 +1,4 @@
+error: comptime `#run` (x) raised an unhandled error: error.Bad
+error return trace (most recent call last):
+  parse at 1037-errors-comptime-run-escape.sx:11:17
+help: handle it at the `#run` site — `#run <expr> catch e { ... }` or `#run <expr> or <default>`

examples/expected/1037-errors-comptime-run-escape.stdout

@@ -0,0 +1 @@
+

examples/expected/1038-errors-comptime-run-handled.exit

@@ -0,0 +1 @@
+164

examples/expected/1038-errors-comptime-run-handled.stderr

@@ -0,0 +1 @@
+

examples/expected/1038-errors-comptime-run-handled.stdout

@@ -0,0 +1,3 @@
+comptime cleanup
+--- build done ---
+ok=10 caught=99 ored=55