Parser now accepts a `..` spread in a parameterized-type arg list; in
instantiateGenericStruct a spread arg bound to the variadic type-param expands
via packTypeElems (so `..sources.T` projects each source pack element protocol
type-arg into ..$Ts). `Combined(s64, ..sources.T)` for a VL(s64) source
instantiates Combined(s64, s64). examples/209 (with explicit per-element xx
erase). 244 examples + unit green.
Next: (..sources) whole-pack materialization with per-element erasure into the
protocol-typed field (c.sources = (..sources) currently segfaults).
e395a08331