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.

examples/150-pack-parse.sx

@@ -1,14 +1,14 @@
-// Variadic heterogeneous type packs — `..$args` — parse lockin.
+// Variadic heterogeneous type packs — `..$args` — parse smoke.
 //
-// First slice of the pack feature: the parser does not yet accept
-// `..$args` (variadic-marker + comptime sigil + name, no type
-// annotation). This test pins the current parse-rejection so the
-// next commit's parser change is visible as a behavior shift.
+// First positive slice of the pack feature: the parser accepts
+// `..$args` (variadic marker + comptime sigil + name) as a
+// parameter declaration. No semantic effect yet — the function
+// is declared but never instantiated; main exists so the example
+// has runnable output.
 //
-// Expected next commit: parser accepts `..$args` and this test
-// either flips to a successful parse (compile error from later
-// passes that haven't been wired up yet) or is replaced by a
-// positive test.
+// Next slices: type-system representation of the pack, impl
+// matching for `Closure(..$args) -> $R`, runtime indexing
+// (`args[$i]`), and the `#insert build_x($args, ...)` pattern.
 
 #import "modules/std.sx";
 
@@ -17,6 +17,6 @@ foo :: (..$args) -> s64 {
 }
 
 main :: () -> s32 {
-    print("never reached\n");
+    print("pack parse ok\n");
     return 0;
 }