ffi M5.A.next.1c.B: pack type rep — Closure(..$args) parses + interns

`parseTypeExpr`'s `Closure(...)` arm now accepts a trailing
`..$name` (sigil optional) as a variadic-pack marker. Pack must
be terminal — `)` is the only token accepted after the name.
`ClosureTypeExpr` AST gains `pack_name: ?[]const u8` carrying the
identifier so later slices can name the binding.

`FunctionInfo` / `ClosureInfo` in src/ir/types.zig grow a
`pack_start: ?u32 = null` field. `Closure(..$args) -> R` interns
as `params = []`, `pack_start = Some(0)` — distinct from any
concrete `Closure(...) -> R` shape thanks to updated hash/eql
arms. New constructor pair `closureTypePack` /
`functionTypePack` keeps the existing single-shape constructors
unchanged.

`type_bridge.resolveClosureType` calls `closureTypePack` when
`pack_name != null`. The pack starts after the fixed prefix,
so `Closure(Prefix, ..$args)` resolves with `params = [Prefix]`,
`pack_start = Some(1)`.

No semantic effect yet — the signature exists in the type table
but no matching code reads `pack_start`. Step 1d wires impl
matching: `Closure(..$args) -> $R` binds against any concrete
closure source type in `tryUserConversion` / `registerParamImpl`.

`examples/154-pack-type-rep.sx` flips from rejecting-with-error
to positive parse smoke (prints "pack type rep ok").

192/192 example tests + `zig build test` green.

src/ir/types.zig

@@ -136,6 +136,11 @@ pub const TypeInfo = union(enum) {
         params: []const TypeId,
         ret: TypeId,
         call_conv: CallConv = .default,
+        /// Pack-variadic shape marker. When set, the signature represents a
+        /// heterogeneous type pack: `params` is the fixed prefix, and a
+        /// per-call-site type list binds the remainder. `pack_start == 0`
+        /// with `params.len == 0` denotes `fn(..$args)`.
+        pack_start: ?u32 = null,
     };
 
     pub const CallConv = enum { default, c };
@@ -143,6 +148,9 @@ pub const TypeInfo = union(enum) {
     pub const ClosureInfo = struct {
         params: []const TypeId,
         ret: TypeId,
+        /// Pack-variadic shape marker — same semantics as FunctionInfo.
+        /// `Closure(..$args) -> $R` => params = [], pack_start = 0.
+        pack_start: ?u32 = null,
     };
 
     pub const OptionalInfo = struct {
@@ -354,11 +362,21 @@ pub const TypeTable = struct {
         return self.intern(.{ .function = .{ .params = owned_params, .ret = ret, .call_conv = cc } });
     }
 
+    pub fn functionTypePack(self: *TypeTable, params: []const TypeId, ret: TypeId, cc: TypeInfo.CallConv, pack_start: u32) TypeId {
+        const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
+        return self.intern(.{ .function = .{ .params = owned_params, .ret = ret, .call_conv = cc, .pack_start = pack_start } });
+    }
+
     pub fn closureType(self: *TypeTable, params: []const TypeId, ret: TypeId) TypeId {
         const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
         return self.intern(.{ .closure = .{ .params = owned_params, .ret = ret } });
     }
 
+    pub fn closureTypePack(self: *TypeTable, params: []const TypeId, ret: TypeId, pack_start: u32) TypeId {
+        const owned_params = self.alloc.dupe(TypeId, params) catch unreachable;
+        return self.intern(.{ .closure = .{ .params = owned_params, .ret = ret, .pack_start = pack_start } });
+    }
+
     pub fn vectorOf(self: *TypeTable, element: TypeId, length: u32) TypeId {
         return self.intern(.{ .vector = .{ .element = element, .length = length } });
     }
@@ -679,10 +697,16 @@ fn hashTypeInfo(h: *std.hash.Wyhash, info: TypeInfo) void {
             h.update(std.mem.asBytes(&f.ret));
             const cc_byte: u8 = @intFromEnum(f.call_conv);
             h.update(&.{cc_byte});
+            const pack_present: u8 = if (f.pack_start != null) 1 else 0;
+            h.update(&.{pack_present});
+            if (f.pack_start) |ps| h.update(std.mem.asBytes(&ps));
         },
         .closure => |c| {
             for (c.params) |p| h.update(std.mem.asBytes(&p));
             h.update(std.mem.asBytes(&c.ret));
+            const pack_present: u8 = if (c.pack_start != null) 1 else 0;
+            h.update(&.{pack_present});
+            if (c.pack_start) |ps| h.update(std.mem.asBytes(&ps));
         },
         .@"struct" => |s| h.update(std.mem.asBytes(&s.name)),
         .@"enum" => |e| h.update(std.mem.asBytes(&e.name)),
@@ -719,6 +743,8 @@ fn typeInfoEql(a: TypeInfo, b: TypeInfo) bool {
                 if (fp != gp) return false;
             }
             if (f.call_conv != g.call_conv) return false;
+            if ((f.pack_start == null) != (g.pack_start == null)) return false;
+            if (f.pack_start) |fp| if (fp != g.pack_start.?) return false;
             return f.ret == g.ret;
         },
         .closure => |c| {
@@ -727,6 +753,8 @@ fn typeInfoEql(a: TypeInfo, b: TypeInfo) bool {
             for (c.params, d.params) |cp, dp| {
                 if (cp != dp) return false;
             }
+            if ((c.pack_start == null) != (d.pack_start == null)) return false;
+            if (c.pack_start) |cp| if (cp != d.pack_start.?) return false;
             return c.ret == d.ret;
         },
         .@"struct" => |s| s.name == b.@"struct".name,