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/cgenstmt.ww

@@ -188,8 +188,8 @@ fn cgreturn(c: *cgen, n: *node) void = {
 					emitline("(BP)\n");
 					zz += 8;
 				};
-				cgwidentaggedstore(c, c.fnret, rhs, scroff,
-				    rsz);
+				cgwidentaggedstore(c, c.fnret, rhs, "BP",
+				    scroff, rsz);
 				emitline("\tMOVQ\t");
 				emitoff(scroff: i64);
 				emitline("(BP), AX\n");
@@ -311,7 +311,7 @@ fn cglet(c: *cgen, n: *node) void = {
 		// ABI call), struct payload (literal/ident), str payload,
 		// scalar payload — with tag remap for tagged-subset widening.
 		if (istaggedtype(c, tn)) {
-			cgwidentaggedstore(c, tn, rhs, off, sz);
+			cgwidentaggedstore(c, tn, rhs, "BP", off, sz);
 			c.lastwasreturn = 0;
 			return;
 		};