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).

examples/241-error-trace-buffer.sx

@@ -24,12 +24,14 @@ propagate :: (n: s32) -> !E {
 }
 
 main :: () -> s32 {
-    // `catch` absorbs the failure → buffer cleared before the handler runs.
+    // `catch` lets the handler INSPECT the trace, then absorbs: the buffer is
+    // cleared when the handler completes (a non-diverging exit), not on entry.
+    // So inside the handler the frames are still visible (here: the `raise` in
+    // `fail` + the `try fail` propagation in `propagate` = 2 frames)...
     propagate(-1) catch e {
-        // The pushes from `raise` + `try` were cleared on catch entry.
-        print("in catch: len={}\n", sx_trace_len());   // 0
+        print("in catch: len={}\n", sx_trace_len());   // 2 (handler sees the chain)
     };
-    print("after catch: len={}\n", sx_trace_len());     // 0
+    print("after catch: len={}\n", sx_trace_len());     // 0 (absorbed at handler exit)
 
     // A success leaves the buffer empty (nothing pushed).
     propagate(1) catch e { };