selfhost+test: route N_DOT base through indexvaluetnode + scanlocals for N_INDEX-lhs cgassign chain (#28+#30)

Wwstage's N_INDEX-lhs cgassign dispatch chain had a triple-site
N_DOT base gap (sister latents filed during #24 / #27 review):

  Read (#28): `obj.mat[i][k]` over a struct field mat: **u8.
  cgindex routes the outer N_INDEX's N_INDEX base through
  indexvaluetnode; the recursion bottomed out at the inner
  N_INDEX's N_DOT base with bt=nil. esz fell through to 8 +
  signed_elem to false — wwstage emitted a stray outer
  `MOVQ $8, CX; IMULQ CX, AX` plus `MOVQ (AX), AX` (8-byte
  read over a 1-byte u8) instead of cstage's bare
  `MOVZBQ (AX), AX`.

  Write (#30): `obj.arr[i] = v` over a struct field arr:
  [N]Tagged (e.g. (i64|str)). cgassign's N_DOT-base arm
  computed esz via indexbaseesz but never set elemtn, so the
  tagged-element store gate missed and the 24-byte tagged slot
  was overwritten by a single scalar MOVQ — wrong-width store
  + tag/payload junk in the upper 16 bytes.

Cstage walks `n->lhs->type` directly via the typed AST
(cmd/w6c/cgen.c idx_eff + the N_INDEX-lhs N_ASSIGN branch).
Wwstage now mirrors via indexvaluetnode, which #24 (aa8ca47)
introduced for the N_INDEX-base case; #28/#30 graduate it for
N_DOT base via the existing dotfieldtnode helper.

Bundle graduates N_DOT base for the entire N_INDEX-lhs cgassign
chain: (a) indexvaluetnode in cgenutil.ww handles N_DOT base via
dotfieldtnode; (b) cgassign N_DOT-base arm in cgenexpr.ww calls
indexvaluetnode for elemtn; (c) scanlocals N_DOT-base arm in
cgendecl.ww parallels the existing N_IDENT arm for tagscr-bump.
Splits are bisect-incoherent: (b)-alone clobbers locals via
under-sized frame, (a)-alone leaves the write path with wrong
elemtn, (c)-alone has no consumer. Only the triple delivers a
complete N_DOT-base graduation matching #24's N_INDEX-base
pattern.

Cstage's first-use+fail-loud strategy for @tagscr (#26 commit
069548d) handles the N_DOT-base shape naturally; the scanlocals
N_DOT arm is wwstage-specific. Long-term rule-10 convergence
(wwstage DOWN from scanlocals to first-use+fail-loud on BOTH
stages) is filed as task #15.

Class A wwstage cgen UNDER. No in-tree consumer; sister latents
filed during #24 + #27 reviews. Test 741_dotbase_chained pins
the dispatch + cstage-byte-identical asm for both rows.

Sister latent (filed): indexbaseesz has no N_TARRAY arm for
scalar struct-field array writes — `s.arr: [N]i32` scalar write
falls through to esz=8 on wwstage. No in-tree exerciser; tight
scope kept here.

115/115 ok. ww2 == ww3 == ww4 byte-id.

selfhost/cmd/wcc/cgendecl.ww

@@ -241,6 +241,12 @@ fn scanlocals(c: *cgen, n: *node) i32 = {
 	// cgwidentaggedstore to materialise the source in before copying
 	// to the element address. Slot is shared per function via
 	// c.tagscrsz (raised to the largest element slot_sz seen).
+	// N_DOT-base graduated (task #30) so `obj.arr[i] = v` for an
+	// [N]Tagged struct field pre-reserves the slot — parallels the
+	// cgassign N_DOT-base elemtn fix in cgenexpr.ww. Without this
+	// arm the runtime localadd allocates past the frame boundary
+	// and clobbers live locals; cstage handles the shape naturally
+	// via first-use+fail-loud (rule-10 convergence filed as #15).
 	if (n.kind == nkind.N_ASSIGN) {
 		let alhs: *node = n.lhs;
 		if (alhs != nil) {
@@ -265,6 +271,21 @@ fn scanlocals(c: *cgen, n: *node) i32 = {
 							};
 						};
 					};
+					if (abase.kind == nkind.N_DOT) {
+						let ft: *node = dotfieldtnode(c, abase);
+						if (ft != nil) {
+							let bk: nkind = ft.kind;
+							let etn: *node = nil;
+							if (bk == nkind.N_TARRAY) { etn = ft.lhs; };
+							if (bk == nkind.N_TSLICE) { etn = ft.lhs; };
+							if (bk == nkind.N_TPTR)   { etn = ft.lhs; };
+							if (etn != nil) {
+								if (istaggedtype(c, etn)) {
+									total += tagscrbump(c, slotsize(c, etn));
+								};
+							};
+						};
+					};
 				};
 			};
 		};