w6c+selfhost: widen concrete variant to tagged-union call arg

Tagged-union widening already fired for `let r: (str|rune) = "...";`,
`r = "...";`, and `return "..."` from a tagged-returning fn — but not
at call sites, so `fn f(x: (str|rune))` couldn't be called with a bare
str or rune. The arg was pushed as its own static type (2 words for
str, 1 for rune) while the callee's slot expected 3 (tag + payload).

C cgen: at the call boundary, look up the callee's declared param
type per arg. When the param is TY_TAGGED and the arg is a concrete
variant, materialise (tag, value-words, padding) sized to the param's
tagged_arg_size — then the existing pop-into-arg-regs logic picks it
up. Nullable `(*T | void)` collapses to a single 8B push.

selfhost: fnret now carries the params head alongside rtype (amalloc
bumped to 48); pushargsrev takes the matching param node and runs the
same widening sequence per arg. The pop drain in cgcall already
handled extra slot words, so no change needed on that side.

Verified with a smoke covering str/rune literals, typed locals,
pre-existing tagged-local pass-through, and nullable widening from a
raw pointer. Selfhost emits byte-identical asm to C cgen on the test.

selfhost/cmd/wcc/cgenutil.ww

@@ -27,9 +27,77 @@ use strconv;
 // the order on the stack so a left-to-right pop into argregs lands
 // (ptr, len) correctly is: PUSHQ BX (top), PUSHQ AX (above) — the
 // pop sequence then yields AX, then BX.
-fn pushargsrev(c: *cgen, arg: *node) i32 = {
+//
+// `param` is the corresponding declared parameter for `arg` (N_PARAM
+// node from the callee's signature) or nil. When param's type is a
+// tagged union and `arg`'s surface type is a concrete variant of it,
+// we materialise (tag, value-words, pad) for the parameter slot before
+// pushing — mirrors cmd/w6c/cgen.c's call-arg widening.
+fn pushargsrev(c: *cgen, arg: *node, param: *node) i32 = {
 	if (arg == nil) { return 0; };
-	let rest: i32 = pushargsrev(c, arg.next);
+	let nextparam: *node = nil;
+	if (param != nil) { nextparam = param.next; };
+	let rest: i32 = pushargsrev(c, arg.next, nextparam);
+	// Implicit widening from a concrete variant to a tagged-union
+	// parameter slot. Skips when the arg is already a tagged local
+	// (line 121's slice-or-tagged shortcut handles that).
+	let widensz: i32 = 0;
+	let widentag: i32 = 0;
+	if (param != nil) {
+		if (param.kind == nkind.N_PARAM) {
+			let ptype: *node = param.lhs;
+			if (istaggedtype(c, ptype)) {
+				let aistagged: bool = false;
+				if (arg.kind == nkind.N_IDENT) {
+					let lc: *local = localfindnode(c, arg.str);
+					if (lc != nil) {
+						aistagged = istaggedtype(c, lc.tnode);
+					};
+				};
+				if (!aistagged) {
+					widensz = slotsize(c, ptype);
+					let tagged: *node = resolvetagged(c, ptype);
+					let t: i32 = taggedvariantindex(c, tagged, arg);
+					if (t < 0) { t = 0; };
+					widentag = t;
+				};
+			};
+		};
+	};
+	if (widensz == 8) {
+		// Nullable fold: pointer value IS the discriminator. No
+		// separate tag word.
+		cgexpr(c, arg);
+		emitline("\tPUSHQ\tAX\n");
+		return rest + 1;
+	};
+	if (widensz > 0) {
+		cgexpr(c, arg);
+		if (nodeisstr(c, arg)) {
+			// slot 24: [+0]=tag,[+8]=ptr,[+16]=len. Push high→low
+			// so pop drains tag first into arg-reg[0].
+			emitline("\tPUSHQ\tBX\n");
+			emitline("\tPUSHQ\tAX\n");
+			emitline("\tMOVQ\t$");
+			emitint(widentag: i64);
+			emitline(", AX\n");
+			emitline("\tPUSHQ\tAX\n");
+		} else {
+			// Scalar variant: single value word at +8. Pad a zero
+			// high word when slot is 24B (some other variant of
+			// the union is 16B-shaped).
+			if (widensz > 16) {
+				emitline("\tXORQ\tDX, DX\n");
+				emitline("\tPUSHQ\tDX\n");
+			};
+			emitline("\tPUSHQ\tAX\n");
+			emitline("\tMOVQ\t$");
+			emitint(widentag: i64);
+			emitline(", AX\n");
+			emitline("\tPUSHQ\tAX\n");
+		};
+		return rest + widensz / 8;
+	};
 	// nkind.N_SLICE expression as arg: `buf[lo:hi]` builds a slice header
 	// on the stack matching C cgen's sequence — push base, push hi,
 	// compute lo, pop into BX/CX, derive len/ptr, push (cap, len, ptr).