lang F1 4.2 (core): generic struct pack type-param + (..$Ts) tuple field

A generic struct can take a pack type-param ..$Ts: []Type that binds the
remaining type args as a sequence, and a pack-shaped tuple field (..$Ts)
resolves to a tuple of those per-position types.

- parser/ast: accept a leading .. on a struct generic param; StructTypeParam
  gains is_variadic.
- registration: TemplateParam carries is_variadic (and is a type param).
- instantiateGenericStruct: a variadic type-param consumes the remaining args
  into pack_bindings + pack_arg_types (mangled into the name); restored after.
- resolveTypeWithBindings: a tuple-literal-as-type containing a pack spread
  (e.g. (..$Ts)) expands via packTypeElems.

Instantiate + correct per-position field types + whole-tuple store + element
read all work (examples/205). Not yet: protocol-applied field (..F(Ts)) (the
canonical (..VL(Ts)) shape) and nested element assignment b.pair.0 = v.
240 examples + unit green.

src/parser.zig

@@ -852,6 +852,13 @@ pub const Parser = struct {
                     try self.expect(.comma);
                     if (self.current.tag == .r_paren) break;
                 }
+                // Optional leading `..` — a pack type-param `..$Ts: []Type`
+                // (must be the last param; binds the remaining type args).
+                var is_variadic = false;
+                if (self.current.tag == .dot_dot) {
+                    is_variadic = true;
+                    self.advance();
+                }
                 // Expect $name : constraint
                 try self.expect(.dollar);
                 if (self.current.tag != .identifier) {
@@ -874,7 +881,7 @@ pub const Parser = struct {
                     }
                 }
                 const pc = try pc_list.toOwnedSlice(self.allocator);
-                try type_params.append(self.allocator, .{ .name = param_name, .constraint = constraint, .protocol_constraints = pc });
+                try type_params.append(self.allocator, .{ .name = param_name, .constraint = constraint, .protocol_constraints = pc, .is_variadic = is_variadic });
             }
             try self.expect(.r_paren);
         }