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.

examples/205-generic-struct-pack-field.sx

@@ -0,0 +1,25 @@
+// Phase 4.2 (core) — a generic struct with a pack type-param `..$Ts: []Type`
+// and a pack-shaped tuple field `(..$Ts)`. Each instantiation binds the
+// remaining type args as the pack, so the field is a tuple of those per-position
+// types. Storing the whole tuple field and reading its elements both work.
+
+#import "modules/std.sx";
+
+Box :: struct($R: Type, ..$Ts: []Type) {
+    r:    $R;
+    pair: (..$Ts);   // tuple of the pack's element types
+}
+
+main :: () -> s32 {
+    // Box(s64, s32, string): R=s64, Ts=[s32, string], pair: (s32, string).
+    a : Box(s64, s32, string) = ---;
+    a.r = 7;
+    a.pair = (42, "hi");                 // whole-tuple field store
+    print("a: r={} 0={} 1={}\n", a.r, a.pair.0, a.pair.1);
+
+    // A different shape → a different per-position tuple field.
+    b : Box(bool, string, bool) = ---;   // Ts=[string, bool], pair: (string, bool)
+    b.pair = ("x", true);
+    print("b: 0={} 1={}\n", b.pair.0, b.pair.1);
+    0;
+}