ERR/E3.3: trace formatting (library/modules/trace.sx) + catch-clear timing fix

The trace formatter, unblocked now that 0057 is fixed.

- library/modules/trace.sx: to_string() walks the trace buffer (sx_trace_len /
  frame_at / truncated) and renders "error return trace ..." with one line per
  frame; print_current() writes it to stderr (libc write(2, ...)). Frame
  locations are "<location pending DWARF>" until E3.0 resolves PCs; count +
  ordering + the overflow note are already meaningful.

- Catch-clear timing fix (lowerCatch): move the absorption clear from
  runCatchBody ENTRY to the handler's non-diverging EXIT (both the pure and
  value-carrying paths). This reconciles the two PLAN-ERR statements that
  conflicted — §clear-points "buffer cleared before the catch body" vs
  §catch-over-or "frames still in the buffer when the body runs". Exit-clear
  satisfies both: the handler can inspect the trace (trace.print_current()
  shows the chain), and the buffer is empty once the handler completes. A
  diverging body (raise/return) keeps/discards on its own path.

- examples/243-trace-format.sx: catch handler prints the tag + the 2-frame
  trace, then shows the buffer is empty after. examples/241 updated: the
  handler now observes len=2 (was 0 under the buggy entry-clear).

Gates: zig build, zig build test, bash tests/run_examples.sh (280 passed; lone
failure is the user's uncommitted 213-canonical-map pack WIP).

library/modules/trace.sx

@@ -0,0 +1,62 @@
+#import "std.sx";
+
+// =====================================================================
+// trace.sx — error return-trace formatting (ERR step E3.3).
+//
+// Reads the thread-local return-trace buffer (ERR E3.1, populated by the
+// push/clear wiring in E3.2) and renders it. A `raise` / propagating `try`
+// pushes a frame; an absorbing site (`catch` / `or value` / destructure)
+// clears the buffer. So at format time the buffer holds exactly the frames
+// of failures that escaped to where you're formatting — typically inside a
+// `catch` handler (the clear fires when the handler completes, so the body
+// still sees the chain) or the (future) failable-`main` wrapper.
+//
+// Frame resolution: a frame is an opaque u64. Resolving it to `file:line:col`
+// needs DWARF line-info (ERR E3.0), which sx does not emit yet — so for now
+// each frame prints as "<location pending DWARF>". The frame COUNT, ordering,
+// and overflow note are already meaningful; once E3.0 lands, only the
+// per-frame location string changes. (The comptime path — resolving a packed
+// `(func_id, ir_offset)` via the interpreter's IR tables — also lands with the
+// resolver in E3.0/E3.3-full.)
+// =====================================================================
+
+libc :: #library "c";
+
+// The error-trace buffer C API (library/vendors/sx_trace_runtime/sx_trace.c),
+// linked in for the JIT and auto-injected for AOT when traces are used.
+sx_trace_len       :: () -> u32 #foreign;
+sx_trace_truncated :: () -> u32 #foreign;
+sx_trace_frame_at  :: (i: u32) -> u64 #foreign;
+
+write :: (fd: s32, buf: [*]u8, count: usize) -> isize #foreign libc;
+
+// Render the current trace buffer to a string (allocated from
+// context.allocator). Empty buffer → "" so callers can cheaply skip output.
+to_string :: () -> string {
+    n := sx_trace_len();
+    if n == 0 { return ""; }
+
+    result := "error return trace (most recent call last):\n";
+    if sx_trace_truncated() != 0 {
+        result = concat(result, "  ... older frames omitted (buffer full)\n");
+    }
+
+    i : u32 = 0;
+    while i < n {
+        frame := sx_trace_frame_at(i);
+        // DWARF (E3.0) will resolve `frame` to file:line:col + function name.
+        // Until then the raw frame value is shown (a placeholder, not a PC yet).
+        line := format("  frame {}: <location pending DWARF> (raw {})\n", i, xx frame);
+        result = concat(result, line);
+        i = i + 1;
+    }
+    result;
+}
+
+// Write the current trace to stderr (fd 2). No-op when the buffer is empty.
+print_current :: () {
+    s := to_string();
+    if s.len > 0 {
+        write(2, s.ptr, xx s.len);
+    }
+}