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.

tests/expected/205-generic-struct-pack-field.exit

@@ -0,0 +1 @@
+0

tests/expected/205-generic-struct-pack-field.txt

@@ -0,0 +1,2 @@
+a: r=7 0=42 1=hi
+b: 0=x 1=true