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/cgenexpr.ww

@@ -3518,6 +3518,16 @@ fn cgassign(c: *cgen, n: *node) void = {
 						};
 					} else { if (base.kind == nkind.N_DOT) {
 						esz = indexbaseesz(c, base);
+						// Without this, the tagged-element gate
+						// below (keyed on elemtn) misses for
+						// `obj.arr[i] = v` over an [N]Tagged field
+						// and the store falls through to scalar —
+						// task #30, sister of the cgindex N_INDEX-
+						// base fix #24. indexvaluetnode now handles
+						// N_DOT base, so the element type drops out
+						// of the same helper.
+						let bt: *node = indexvaluetnode(c, lhs);
+						if (bt != nil) { elemtn = bt; };
 					} else { if (base.kind == nkind.N_INDEX) {
 						// Chained-write write-side parallel of the
 						// cgindex N_INDEX-base arm graduated in #24: