selfhost+test: route chained N_INDEX outer element size through indexvaluetnode (#24)

Wwstage cgindex's base-inspection (cgenexpr.ww) only computed esz/
signed_elem when base.kind == N_IDENT or N_DOT. For a chained
`names[i][k]` (names: **u8) the outer N_INDEX has base.kind ==
N_INDEX; esz fell through to the default 8 so the outer load
emitted `MOVQ (AX), AX` over a 1-byte u8 plus a stray
`MOVQ $8, CX; IMULQ CX, AX` scaling on the outer index that cstage
doesn't emit. Wrong-width-narrow-load: the byte was read as 8 bytes
(reaching into adjacent memory) and the outer offset multiplied by
sizeof *u8 instead of sizeof u8.

Cstage walks n->lhs->type directly via the typed AST
(cmd/w6c/cgen.c idx_eff → eff->sub->size at N_INDEX). Wwstage
needed the parallel via indexvaluetnode — return the value-type
of an N_INDEX expression by stripping one element layer off base's
type, recursing for chained inner. cgindex's else-if chain now
adds the N_INDEX arm: call indexvaluetnode + elemsizeofc/
elemissignedc.

Class A wwstage cgen UNDER. Surfaced first time the codebase
exercised the **T[i][k] shape — through expanddir in
selfhost/cmd/ww/main.ww (post-#22 dir-enum, commit 9e0816e). The
workaround there split names[i][k] into `let nm: *u8 = names[i];
nm[k]` to route through the bare-pointer index path. Retired in
this commit: expanddir uses the natural chained form since the
read path is now byte-identical across stages.

Bundling justification (rule 11): the workaround retirement is
the in-tree verification this fix works — without retiring,
neither bootstrap byte-id nor 995_self_rebuild exercises the
chained read shape. Test 739_chained_index pins cstage-byte-
identical asm for **u8 (MOVZBQ load, 1 inner-stride-8 IMULQ pair,
no outer scale) + **i32 (MOVSXD load, inner $8 + outer $4 IMULQ
pairs).

Sister latents filed (no in-tree consumer, no probe):
  Task #27 — cgassign chained-write N_INDEX: write path
  `names[i][k] = v` for **u8 has the same dispatch gap. Selfhost +
  lib grep is empty.
  New latent (filed during review) — cgindex N_DOT base on chained
  index: `obj.mat[i][k]` over a struct-field base falls back to
  esz=8. indexvaluetnode currently handles N_IDENT + N_INDEX bases
  only.

113/113 ok. ww2 == ww3 == ww4 byte-id.

Makefile

@@ -266,6 +266,7 @@ TESTS	= $(BIN)/test_smoke $(BIN)/test_lex $(BIN)/test_parse $(BIN)/test_check \
 	  $(BIN)/test_cstage_label_ssot \
 	  $(BIN)/test_direnum \
 	  $(BIN)/test_module_decl \
+	  $(BIN)/test_chained_index \
 	  $(BIN)/test_fnparams_bare_leaf_shadow \
 	  $(BIN)/test_fnret_bare_leaf_shadow \
 	  $(BIN)/test_param_shadow_mod \
@@ -639,6 +640,10 @@ $(BIN)/test_direnum: test/wcc/737_direnum.c \
 $(BIN)/test_module_decl: test/wcc/738_module_decl.c $(LIB)/libwcc.a | $(BIN)
 	$(CC) $(CFLAGS) -Icmd/wcc -o $@ $< -Lout/lib -lwcc
 
+$(BIN)/test_chained_index: test/wcc/739_chained_index.c \
+		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
+	$(CC) $(CFLAGS) -o $@ $<
+
 $(BIN)/test_fnparams_bare_leaf_shadow: test/wcc/732_fnparams_bare_leaf_shadow.c \
 		$(BIN)/w6c $(BIN)/w6c_ww | $(BIN)
 	$(CC) $(CFLAGS) -o $@ $<