w6c+selfhost: cg_widen_tagged_store basereg + N_ASSIGN tagged field (closes #26)

Extended cg_widen_tagged_store (cstage) / cgwidentaggedstore (wwstage)
to take a base_reg/basereg parameter so the primitive supports non-BP
destinations. Cstage extends body in-place via via_outer gate +
spill+scratch+copy-out; wwstage splits into wrapper (non-BP) +
cgwidentaggedstorebp (BP-only) to dodge the no-goto constraint. New
N_ASSIGN field TY_TAGGED branch routes through the primitive for all
rhs shapes.

Scope-adjacent: fieldsize recurses through N_TTAGGED via slotsize and
TNAME-aliased-to-tagged via aliaslookup. Needed for the test fixtures.

Wwstage read-side N_DOT-of-tagged-field source is filed as task #28;
test rows use mark-canary verification until that lands.

selfhost/cmd/wcc/cgendecl.ww

@@ -204,6 +204,61 @@ fn scanlocals(c: *cgen, n: *node) i32 = {
 			};
 		};
 	};
+	// Tagged-union struct-field write: `s.f = v` or `(*p).f = v`
+	// where f is a tagged-union field. cgassign delegates to
+	// cgwidentaggedstore; for pointer-rooted dst the wrapper
+	// allocates @tagbase (8B) and @tagscr (slot_sz). Both names
+	// dedup with other tagged scratch users in the same function.
+	if (n.kind == nkind.N_ASSIGN) {
+		let alhs: *node = n.lhs;
+		if (alhs != nil) {
+			if (alhs.kind == nkind.N_DOT) {
+				let abase: *node = alhs.lhs;
+				if (abase != nil) {
+					if (abase.kind == nkind.N_IDENT) {
+						let lc: *local = localfindnode(c, abase.str);
+						let btn: *node = nil;
+						if (lc != nil) { btn = lc.tnode; }
+						else           { btn = letvartnode(c, abase.str); };
+						let isptr: bool = false;
+						let stype: *node = nil;
+						if (btn != nil) {
+							if (btn.kind == nkind.N_TPTR) {
+								isptr = true;
+								stype = btn.lhs;
+							};
+							if (btn.kind == nkind.N_TNAME) { stype = btn; };
+						};
+						if (stype != nil) {
+							if (stype.kind == nkind.N_TNAME) {
+								let si: *structinfo = structlookup(c, stype.str);
+								if (si != nil) {
+									let fi: *fieldinfo = si.fields;
+									for (fi != nil) {
+										if (streq(fi.fname, alhs.str)) {
+											if (istaggedtype(c, fi.tnode)) {
+												if (isptr) {
+													if (!scanseenmark(c, "@tagbase")) {
+														total += 8;
+													};
+													if (!scanseenmark(c, "@tagscr")) {
+														total += 24;
+													};
+												};
+											};
+											fi = nil;
+										} else {
+											fi = fi.finext;
+										};
+									};
+								};
+							};
+						};
+					};
+				};
+			};
+		};
+	};
 	// Tagged-union return with struct payload or tagged-subset
 	// source — cgreturn materialises in @tagscr then loads
 	// AX/DX/CX. Detect via the same rhsstructpayload predicate