fix(0108): break/continue run the loop body's pending defers

lowerBreak/lowerContinue emitted a bare br, and the enclosing block's
emitBlockDefers — seeing the terminator — discarded the pending entries
on the assumption a return had already drained them. The breaking
iteration's defers were silently skipped, leaking whatever the cleanup
released.

Lowering.loop_defer_base records the defer-stack height at each loop's
body start (while / for / range-for, saved and restored alongside
break_target); break/continue drain non-onfail entries down to it in
LIFO order via the non-truncating emitLoopExitDefers before branching.
Truncation stays with the lexical block exits — the same entries still
belong to the fall-through path after the branch containing the break.
break/continue outside a loop now diagnose instead of no-op'ing.

Regression: examples/0049-basic-defer-break-continue.sx (for and while,
break and continue, nested-block LIFO drain).

examples/expected/0049-basic-defer-break-continue.stdout

@@ -0,0 +1,18 @@
+body 0
+cleanup 0
+cleanup 1
+after break loop
+b2 0
+c2 0
+c2 1
+b2 2
+c2 2
+done
+wbody1
+w1
+w2
+w3
+while done
+inner 0
+outer 0
+nested done