ERR/E4.1: trace.print_interpreter_frames() — Phase E4 complete

The last E4 item: a comptime call-frame dump.

- New nullary `interp_print_frames` IR op (inst/print). The interpreter
  maintains a `call_chain` side-stack (push/pop a FuncId around each sx-bodied
  `call`, freed in deinit) and `printInterpFrames` appends the chain to its
  output — most-recent-last, with the dump frame itself skipped. emit_llvm
  makes the op a no-op: compiled code has no interpreter stack, and the only
  caller is `process.exit`'s dead `is_comptime()` branch.
- Lowered from a name-recognized `__interp_print_frames()` builtin
  (tryLowerReflectionCall + inferExprType → void).
- `trace.print_interpreter_frames()` wraps the builtin; wired into
  `process.exit`'s comptime branch (process.sx now imports trace.sx).
- Frame source locations await IR-offset resolution (the comptime analog of
  DWARF), so only function names print today.

examples/252-interp-frames.sx (top-level `#run` drives the dump; exit 0).
Phase E4 (entry-point + stdlib error story) is now 100% complete.

examples/252-interp-frames.sx

@@ -0,0 +1,23 @@
+// `trace.print_interpreter_frames()` (ERR step E4.1). At comptime (`#run`) it
+// dumps the interpreter's active sx call-frame chain (most recent call last) to
+// the build output; in compiled code it folds to nothing (no interpreter stack
+// — the only real caller is `process.exit`'s dead `is_comptime()` branch). The
+// dump frame itself is omitted; frame source locations await IR-offset
+// resolution, so only names print today. Expected exit: 0.
+
+#import "modules/std.sx";
+trace :: #import "modules/trace.sx";
+
+probe :: () {
+    trace.print_interpreter_frames();    // dumps the chain: __run_0 → inner → probe
+}
+
+inner :: () {
+    probe();
+}
+
+#run inner();                             // top-level #run drives the chain
+
+main :: () -> s32 {
+    return 0;
+}