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).

src/ir/lower.zig

@@ -15678,7 +15678,16 @@ pub const Lowering = struct {
             self.builder.condBr(is_err, handle_bb, &.{}, merge_bb, &.{});
             self.builder.switchToBlock(handle_bb);
             _ = self.runCatchBody(ce, err_val, err_set, null);
-            if (!self.currentBlockHasTerminator()) self.builder.br(merge_bb, &.{});
+            // The handler can inspect the trace (`trace.print_current()`); the
+            // absorption clear fires once it completes WITHOUT re-raising (a
+            // fall-through). A diverging body (`raise` / `return`) keeps /
+            // discards the buffer on its own path (ERR E3.2; reconciles
+            // PLAN-ERR §clear-points "cleared before body" with §catch-over-or
+            // "frames still in the buffer when the body runs").
+            if (!self.currentBlockHasTerminator()) {
+                self.emitTraceClear();
+                self.builder.br(merge_bb, &.{});
+            }
             self.builder.switchToBlock(merge_bb);
             return self.builder.constInt(0, .void);
         }
@@ -15715,6 +15724,9 @@ pub const Lowering = struct {
                 }
                 break :blk self.builder.constUndef(succ_ty);
             };
+            // Absorption clear on a non-diverging handler (see the pure-failable
+            // path above): the body saw the trace, now it's consumed.
+            self.emitTraceClear();
             self.builder.br(merge_bb, &.{bv});
         }
 
@@ -15727,10 +15739,6 @@ pub const Lowering = struct {
     /// catch), returns the body's value (or null if the body diverged); when
     /// null (pure-failable catch), runs the body for effect and returns null.
     fn runCatchBody(self: *Lowering, ce: *const ast.CatchExpr, err_val: Ref, err_set: TypeId, want_ty: ?TypeId) ?Ref {
-        // `catch` absorbs the LHS's failure: clear the trace buffer before the
-        // handler runs (ERR E3.2), so a failure consumed here leaves no residue.
-        // Runs on the error/handle path only (this fn is called from handle_bb).
-        self.emitTraceClear();
         var handle_scope = Scope.init(self.alloc, self.scope);
         const saved_scope = self.scope;
         self.scope = &handle_scope;