lang 1.1: parse pack-constrained variadic parameter

`..xs: Protocol` (a bare protocol, no `[]`, no `$`) on a variadic
parameter now parses to `ast.Param.is_pack = true` — a heterogeneous
protocol-constrained pack, distinct from a slice variadic
(`..xs: []T`, is_pack=false) and the comptime type-pack (`..$args`,
is_comptime=true). Parser-only: sema/lowering for the pack form land in
Phase 2; existing forms are unaffected (zero examples used a bare
non-slice variadic annotation). Adds three parser unit tests and
examples/probes/pack-param-parses.sx.

src/ast.zig

@@ -127,6 +127,12 @@ pub const Param = struct {
     type_expr: *Node,
     is_variadic: bool = false,
     is_comptime: bool = false,
+    /// Heterogeneous protocol-constrained variadic pack: `..xs: Protocol`
+    /// (no `[]`, no `$`). The annotation is a bare protocol the trailing args
+    /// each conform to with their own type-arg — distinct from a slice variadic
+    /// (`..xs: []T`, `is_pack == false`) and from the comptime type-pack
+    /// (`..$xs`, `is_comptime == true`). Always implies `is_variadic`.
+    is_pack: bool = false,
     /// Optional default value expression. When the caller omits this
     /// parameter, lowering substitutes this expression in its place.
     default_expr: ?*Node = null,