wcc: continue runs post-step in 3-clause for and range form (#138)

`for (init; cond; post) { ... continue; ... }` and `for (let i .. xs)
{ ... continue; ... }` now emit a `post` (3-clause) or `rpost` (range)
label between the body and the JMP back to the cond-test. `continue`
jumps to that label, runs the post-step, then re-tests the loop
condition — mirrors C/Go/Hare semantics. Pre-fix both stages emitted
`JMP loop_top` for continue, SKIPPING the post-step → the value that
triggered continue never advanced → silent infinite loop on the first
matching iteration. Found by impl-strconv-fold2 during the fold-3
decimal.ha port: `leftshift_newdigits`'s `for (... i+=1) { ... else
if (d.digits[i]==p5[i]) continue; ... }` would infinite-loop at the
first equal digit.

BOTH stages were identically buggy → 990-997 cs==ww byte-id held →
gate-blind. Bootstrap audit (`grep -rE 'for \(let .*\.\.' lib/
selfhost/`) confirmed zero existing callers with continue in either
the 3-clause or range form; bootstrap-NEUTRAL.

Sites: cmd/w6c/cgen.c N_FOR + N_FORRANGE; selfhost/cmd/wcc/
cgenstmt.ww cgfor + cgforrange. 1-clause `for (cond)` byte-id
preserved (cont_target stays = loop_top when n.rhs == nil). Rule-11
carve-out: 3-clause and range share the lowered structure; fixing
one without the other would leave the same silent miscompile in
N_FORRANGE — one-class closure on the continue-skips-post bug, same
precedent as #133-expanded.

911_continue_run: 4 rows. for3_skip_one (lead's repro, was infinite
loop, now 4), for3_skip_two (nested continues, 30), range_skip
(Hare-range continue, was infinite loop, now 120), for1_continue_
byteid (1-clause regression assertion — bootstrap shape unchanged).
Pre-existing parser-side divergences (cstage silently drops post in
the never-used 2-clause `for (cond; post)`; wwstage doesn't support
infinite `for {}`) deferred to #139 — not in decimal.ha, no shared
class with the cgen continue-skips-post.

Makefile

@@ -332,6 +332,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_deref_narrow_run \
 	  $(BIN)/test_idx_compound_run \
 	  $(BIN)/test_dotbase_arr_run \
+	  $(BIN)/test_continue_run \
 	  $(BIN)/test_f64cgen_run \
 	  $(BIN)/test_f64crossmod_run \
 	  $(BIN)/test_tuprecv_run \
@@ -1106,6 +1107,11 @@ $(BIN)/test_dotbase_arr_run: test/wcc/949_dotbase_arr_run.c $(BIN)/ww \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_continue_run: test/wcc/911_continue_run.c $(BIN)/ww \
+		$(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_f64cgen_run: test/wcc/951_f64cgen_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<