selfhost: cgfnparams variadic T... straddle stitch (closes #12)

Last reg/stack-straddle gap closed — variadic T... now stitches at
idx=5/regs_left=1/nw=3. Four classes (tagged/slice/str/variadic) at
cgendecl.ww 467/518/564/394 are now structurally symmetric. cgfn
pre-scan predicate widened to (istg || issl || isst || isvar).
cstage unchanged — type-promoted T... → []T already hits the slice
partial-fit branch.

selfhost/cmd/w6c/main.combined.ww

@@ -13910,10 +13910,37 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					emitoff((off + 16): i64);
 					emitline("(BP)\n");
 					idx += 1;
+				} else { if (idx < 6) {
+					// Partial-fit stitch — variadic `T...` is a slice
+					// at the ABI boundary (the call site synthesises a
+					// 24B descriptor and pushes ptr/len/cap), so this
+					// mirrors the slice branch at cgendecl.ww:518.
+					let off: i32 = localadd(c, nm, 24, tn);
+					let regs_left: i32 = 6 - idx;
+					let w: i32 = 0;
+					for (w < regs_left) {
+						emitline("\tMOVQ\t");
+						emitline(argregname(idx));
+						emitline(", ");
+						emitoff((off + w*8): i64);
+						emitline("(BP)\n");
+						idx += 1;
+						w += 1;
+					};
+					for (w < 3) {
+						emitline("\tMOVQ\t");
+						emitoff((16 + stkcursor*8): i64);
+						emitline("(BP), AX\n");
+						emitline("\tMOVQ\tAX, ");
+						emitoff((off + w*8): i64);
+						emitline("(BP)\n");
+						stkcursor += 1;
+						w += 1;
+					};
 				} else {
 					localaddstack(c, nm, tn, 16 + stkcursor*8);
 					stkcursor += 3;
-				};
+				};};
 				p = p.next;
 				continue;
 			};
@@ -14180,15 +14207,13 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 				frame += sz;
 				if (isf) { fargi += 1; }
 				else { argi += eb; };
-			} else { if (eb > 1 && regs_left > 0 && (istg || issl || isst)) {
+			} else { if (eb > 1 && regs_left > 0 && (istg || issl || isst || isvar)) {
 				// Multi-word param straddles the reg/stack boundary;
 				// cgfnparams stitches the tail from positive BP
 				// offsets into a single local slot, so we still
 				// reserve the full size. Symmetric across tagged,
-				// slice and str (cgendecl.ww:440/491/537). Variadic
-				// `T...` is deliberately omitted (task #12) — the
-				// cgfnparams variadic branch doesn't stitch yet, so
-				// it falls through to the pure-stack accounting.
+				// slice, str and variadic `T...`
+				// (cgendecl.ww:467/518/564/394).
 				frame += sz;
 				argi = 6;
 			} else {

selfhost/cmd/wcc/cgendecl.ww

@@ -391,10 +391,37 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					emitoff((off + 16): i64);
 					emitline("(BP)\n");
 					idx += 1;
+				} else { if (idx < 6) {
+					// Partial-fit stitch — variadic `T...` is a slice
+					// at the ABI boundary (the call site synthesises a
+					// 24B descriptor and pushes ptr/len/cap), so this
+					// mirrors the slice branch at cgendecl.ww:518.
+					let off: i32 = localadd(c, nm, 24, tn);
+					let regs_left: i32 = 6 - idx;
+					let w: i32 = 0;
+					for (w < regs_left) {
+						emitline("\tMOVQ\t");
+						emitline(argregname(idx));
+						emitline(", ");
+						emitoff((off + w*8): i64);
+						emitline("(BP)\n");
+						idx += 1;
+						w += 1;
+					};
+					for (w < 3) {
+						emitline("\tMOVQ\t");
+						emitoff((16 + stkcursor*8): i64);
+						emitline("(BP), AX\n");
+						emitline("\tMOVQ\tAX, ");
+						emitoff((off + w*8): i64);
+						emitline("(BP)\n");
+						stkcursor += 1;
+						w += 1;
+					};
 				} else {
 					localaddstack(c, nm, tn, 16 + stkcursor*8);
 					stkcursor += 3;
-				};
+				};};
 				p = p.next;
 				continue;
 			};
@@ -661,15 +688,13 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 				frame += sz;
 				if (isf) { fargi += 1; }
 				else { argi += eb; };
-			} else { if (eb > 1 && regs_left > 0 && (istg || issl || isst)) {
+			} else { if (eb > 1 && regs_left > 0 && (istg || issl || isst || isvar)) {
 				// Multi-word param straddles the reg/stack boundary;
 				// cgfnparams stitches the tail from positive BP
 				// offsets into a single local slot, so we still
 				// reserve the full size. Symmetric across tagged,
-				// slice and str (cgendecl.ww:440/491/537). Variadic
-				// `T...` is deliberately omitted (task #12) — the
-				// cgfnparams variadic branch doesn't stitch yet, so
-				// it falls through to the pure-stack accounting.
+				// slice, str and variadic `T...`
+				// (cgendecl.ww:467/518/564/394).
 				frame += sz;
 				argi = 6;
 			} else {

selfhost/cmd/wwdump/main.combined.ww

@@ -13910,10 +13910,37 @@ fn cgfnparams(c: *cgen, params: *node) void = {
 					emitoff((off + 16): i64);
 					emitline("(BP)\n");
 					idx += 1;
+				} else { if (idx < 6) {
+					// Partial-fit stitch — variadic `T...` is a slice
+					// at the ABI boundary (the call site synthesises a
+					// 24B descriptor and pushes ptr/len/cap), so this
+					// mirrors the slice branch at cgendecl.ww:518.
+					let off: i32 = localadd(c, nm, 24, tn);
+					let regs_left: i32 = 6 - idx;
+					let w: i32 = 0;
+					for (w < regs_left) {
+						emitline("\tMOVQ\t");
+						emitline(argregname(idx));
+						emitline(", ");
+						emitoff((off + w*8): i64);
+						emitline("(BP)\n");
+						idx += 1;
+						w += 1;
+					};
+					for (w < 3) {
+						emitline("\tMOVQ\t");
+						emitoff((16 + stkcursor*8): i64);
+						emitline("(BP), AX\n");
+						emitline("\tMOVQ\tAX, ");
+						emitoff((off + w*8): i64);
+						emitline("(BP)\n");
+						stkcursor += 1;
+						w += 1;
+					};
 				} else {
 					localaddstack(c, nm, tn, 16 + stkcursor*8);
 					stkcursor += 3;
-				};
+				};};
 				p = p.next;
 				continue;
 			};
@@ -14180,15 +14207,13 @@ fn cgfn(c: *cgen, fn_: *node) void = {
 				frame += sz;
 				if (isf) { fargi += 1; }
 				else { argi += eb; };
-			} else { if (eb > 1 && regs_left > 0 && (istg || issl || isst)) {
+			} else { if (eb > 1 && regs_left > 0 && (istg || issl || isst || isvar)) {
 				// Multi-word param straddles the reg/stack boundary;
 				// cgfnparams stitches the tail from positive BP
 				// offsets into a single local slot, so we still
 				// reserve the full size. Symmetric across tagged,
-				// slice and str (cgendecl.ww:440/491/537). Variadic
-				// `T...` is deliberately omitted (task #12) — the
-				// cgfnparams variadic branch doesn't stitch yet, so
-				// it falls through to the pure-stack accounting.
+				// slice, str and variadic `T...`
+				// (cgendecl.ww:467/518/564/394).
 				frame += sz;
 				argi = 6;
 			} else {