ffi M5.A.next.1b: parser accepts ..$args as a variadic-pack param

Extends parseParams in src/parser.zig:1558 to recognize a leading
`..` before the optional `$` sigil and the parameter name. The
old `args: ..T` form (variadic marker after the colon) still
works — both paths set the same `is_variadic` flag.

A pack declaration `..$args` parses as:
- `is_variadic = true` (from the leading `..`)
- `is_comptime = true` (from the `$` sigil)
- `type_expr = inferred_type` (no `:` annotation)

The no-colon branch now propagates `is_variadic` and `is_comptime`
onto the Param struct so later slices (type rep, impl matching,
monomorphisation) can read both flags from the parsed AST without
re-deriving from token sequence.

`examples/150-pack-parse.sx` flips from rejecting-with-error to
positive parse smoke. No semantic effect yet — `foo` is declared
but never instantiated.

191/191 example tests + `zig build test` green.

tests/expected/150-pack-parse.exit

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

tests/expected/150-pack-parse.txt

@@ -1 +1 @@
-/Users/agra/projects/sx/examples/150-pack-parse.sx:15:9: error: expected parameter name
+pack parse ok