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/ast.zig

@@ -328,6 +328,9 @@ pub const StructTypeParam = struct {
     name: []const u8, // e.g. "N" or "T" (without $)
     constraint: *Node, // type_expr: "u32" for value param, "Type" for type param
     protocol_constraints: []const []const u8 = &.{}, // e.g. ["Eq", "Hashable"] for $T/Eq/Hashable
+    /// `..$Ts: []Type` — a pack type-param binding the remaining type args as a
+    /// sequence (must be last). Field types reference it via `(..$Ts)` etc.
+    is_variadic: bool = false,
 };
 
 pub const UsingEntry = struct {