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.

library/modules/process.sx

@@ -1,4 +1,5 @@
 #import "std.sx";
+trace :: #import "trace.sx";
 
 // =====================================================================
 // process.sx — subprocess + environment stdlib (POSIX backend).
@@ -136,6 +137,7 @@ clib_exit :: (code: s32) -> noreturn #foreign libc "_exit";
 exit :: (code: u8, loc: Source_Location = #caller_location) -> noreturn {
     if is_comptime() {
         print("\nprocess.exit({}) called from {} at {}:{}\n", code, loc.func, loc.file, loc.line);
+        trace.print_interpreter_frames();
     }
     clib_exit(xx code);
 }

library/modules/trace.sx

@@ -60,3 +60,12 @@ print_current :: () {
         write(2, s.ptr, xx s.len);
     }
 }
+
+// Dump the comptime (`#run`) interpreter call-frame chain (ERR E4.1). At
+// comptime the interpreter walks its active sx frames and appends them to the
+// build output; in compiled code this folds to nothing (there is no
+// interpreter stack — the only caller is a dead `is_comptime()` branch).
+// Frame source locations await IR-offset resolution, so only names print today.
+print_interpreter_frames :: () {
+    __interp_print_frames();
+}