wcc+w6c+w6c_ww: insert() builtin — single-element slice insertion (part of #35)

Hare's insert(xs[idx], v) (ref/harec/src/check.c:745
check_expr_append_insert — append/insert share the checker arm,
"insert" at :786): checker accepts an INDEX place over a slice plus
one value, stamps void; idx == len is a legal end-insert (the
ref/hare os/exec/platform_cmd.ha:86 idiom). Loud-rejects with exact
texts: spread form insert(xs[i], vs...) (filed, #35 — also covers
harec's with-length form via the arity check), range place (not
Hare; harec only parses ACCESS_INDEX, :784), non-index operands,
array bases, wrong arity. delete()-parity throughout.

Lowering (both stages, converged byte-identical by construction) is
a DESUGAR: append(xs, v) — reusing append's grow (rt_ensure) and the
entire #34 value-store dispatch (scalar / str-slice header / tagged
widen / struct fill) verbatim, one boxing choke-point — lands v at
slot len-1; then a rotate-right of [idx, len) moves it home through
a fresh per-site esz frame scratch (@insscr). The rotate is delete's
shift loop in reverse (descending j, the safe memmove-up direction)
and is a same-slice whole-stride raw byte move — no boxing exists
for any element kind. idx evaluates BEFORE the grow (Hare's
left-to-right operand order — pinned by the pregrow_len_idx row,
insert(xs[len(xs)-1], v): pre-grow [7,13,11] vs post-grow [7,11,13];
an idx==len(xs) end-insert cannot discriminate, the rotate
degenerates either way). Base shapes: local slice ident (LEAQ) and
deref-of-local ptr-to-slice (MOVQ); others rule-7 loud-stop, like
delete.

test/807: 57 fixtures — front/middle/end + idx==len via len(xs) +
the pre-grow eval-order pin, esz 1/2/4/8/16/24/56 (MOVB/MOVW/MOVL
tails, struct body, str header, 7-qword tagged from a typed local
[the regex fold-3 ha:347 newinst shape] and from a cast rvalue
[ha:419/441]), empty-slice grow, (*p)[i] deref base, front-insert
loop, 6 checker reject rows with diagnostic-text checks; every
accept row cs==ww asm byte-id.

Makefile

@@ -390,6 +390,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_tuple_sret_receive_run \
 	  $(BIN)/test_append_wide_elem \
 	  $(BIN)/test_delete_elem \
+	  $(BIN)/test_insert_elem \
 	  $(BIN)/test_placeaddr_store \
 	  $(BIN)/test_tryprop_multisuccess \
 	  $(BIN)/test_append_place \
@@ -1015,6 +1016,19 @@ $(BIN)/test_delete_elem: test/wcc/804_delete_elem.c \
 		$(LIB)/libwwrt.a | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<
 
+# #35 (insert-half): insert(xs[idx], v) single-element slice insertion —
+# delete()'s twin, lowered as append-desugar + rotate-right. Runtime rows
+# across every element kind (scalar/narrow/str/struct/56B tagged incl.
+# the regex fold-3 typed-local + cast-rvalue consumer shapes) + idx==len
+# end-insert + pre-grow idx evaluation + the deref-of-local base +
+# checker reject rows w/ exact diagnostic text + cs==ww asm byte-id
+# (the converged-by-construction rotate loop + @insscr frame slot).
+$(BIN)/test_insert_elem: test/wcc/807_insert_elem.c \
+		$(BIN)/ww $(BIN)/w6c $(BIN)/w6c_ww $(BIN)/w6a $(BIN)/w6l \
+		$(BIN)/ww_ww \
+		$(LIB)/libwwrt.a | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 # F6 (task #4, regex fold-2b): (*ts)[i].field = v / OP= v through the
 # cgplaceaddr resolver — runtime rows across widths/signedness/str +
 # compound ops + loud-tail reject rows w/ exact diagnostic text + cs==ww