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.

src/ir/print.zig

@@ -144,6 +144,7 @@ fn printInst(instruction: *const Inst, ref_idx: u32, tt: *const TypeTable, write
         .const_null => try writer.writeAll("const null : "),
         .const_undef => try writer.writeAll("const undef : "),
         .is_comptime => try writer.writeAll("is_comptime : "),
+        .interp_print_frames => try writer.writeAll("interp_print_frames : "),
         .const_type => |tid| try writer.print("const type({s}) : ", .{tt.typeName(tid)}),
 
         // ── Arithmetic ──────────────────────────────────────────