wcc: #152 let-initializer scope — defer the binding's localfind link past its own init (both stages)

A let's own name was visible during its OWN initializer: cgen prepended the
new local into the name-keyed localfind chain BEFORE emitting the init, so
`let x = f(x)` read the fresh UNINIT slot, not the outer/param x. Both-wrong-
identical silent miscompile (gate-blind byte-id). Surfaced by path
dirname/basename (was the c3-posix path->p rename).

Align to Hare (harec check.c:1439 evals the init, then scope_insert). Fix,
both stages, IDENTICAL asm: reserve the frame slot BEFORE the init emits,
link the binding's name into the localfind chain only AFTER.
- cstage cgen.c: split localoff -> localslot(reserve)+link; N_LET's 12
  case-level breaks -> goto letlink (tail links once); the inner-for break
  is preserved; the 4 fatal() arms untouched.
- wwstage cgen.ww/cgenstmt.ww: new localreserve (= localalloc minus the
  chain-link); cglet -> cgletbody(c,n,off) + a cglet wrapper that
  reserves -> calls body -> links after.

Byte-id-safe on existing code: localfind is by-name, so deferring the link
is a no-op on every non-self-shadow let (grep = 0 self-shadow sites) — 990-997
stay green. Because both stages emit identical now-correct asm, byte-id
CANNOT catch this; the pin is a RUNTIME test, teeth-proven (revert -> pin
fails). test/wcc/989_letshadow{.ww,_run.c}: param-shadow, let-in-init shadow,
rename control, arrlit self-ref.

Embedded regen: selfhost/cmd/{w6c,wwdump}/main.combined.ww. Gate: all 325
passed, byte-id 990-997 green, w6c c587f4a1 / w6c_ww 7a69f898 (deterministic).

Makefile

@@ -448,6 +448,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_strings_run \
 	  $(BIN)/test_hex_run $(BIN)/test_utf8_run $(BIN)/test_bytes_run \
 	  $(BIN)/test_path_run \
+	  $(BIN)/test_letshadow_run \
 	  $(BIN)/test_decimal_run $(BIN)/test_strconv_int_run \
 	  $(BIN)/test_stof_run $(BIN)/test_ftos_run \
 	  $(BIN)/test_memio_run $(BIN)/test_temp_run $(BIN)/test_getopt_run \
@@ -1997,6 +1998,10 @@ $(BIN)/test_path_run: test/wcc/989_path_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+$(BIN)/test_letshadow_run: test/wcc/989_letshadow_run.c $(BIN)/ww $(BIN)/w6c \
+		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_ascii_run: test/wcc/904_ascii_run.c $(BIN)/ww $(BIN)/w6c \
 		$(BIN)/w6a $(BIN)/w6l $(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<