ffi M5.A.next.4A.bare.1.B: bare $args lowers to []Type slice value

Step 4A final-slice fix. Bare `$<pack_name>` (no `[<int>]`)
in expression position now parses + lowers to a comptime
`[]Type` slice value carrying one `const_type(TypeId)` per
pack element.

Plumbing:

- src/ast.zig: new `ComptimePackRef { pack_name }` node +
  `comptime_pack_ref` variant in Data.
- src/parser.zig: `parsePrimary`'s `$` arm makes `[` optional
  after the pack name. With `[<int>]` → existing
  `pack_index_type_expr` (single Type value). Without → new
  `comptime_pack_ref` (whole pack as []Type).
- src/sema.zig: adds the no-op switch arms for the new node
  in `analyzeNode` and `findNodeAtOffset`.
- src/ir/lower.zig: `lowerExpr` arm reads `pack_arg_types[name]`
  and calls `buildPackSliceValue(arg_tys)`. The helper allocas
  a `[N x Any]` array, emits one `const_type(arg_tys[i])` per
  slot, then a slice `{data_ptr, len}` aggregate. No active
  binding → focused diagnostic + null slice placeholder. The
  IR slice element type is `Any` (matches the today's
  `Type → .any` mapping in type_bridge); the interp stores
  raw `.type_tag` Values directly (NOT Any-boxed) so
  `args[i]` at interp time reads a Type value.
- src/ir/emit_llvm.zig: relaxed `const_type` to silently emit
  undef-i64 instead of the previous stderr-noisy bail. Storage
  of Type values in runtime aggregates is harmless (undef in,
  undef out). Use-site misuse is caught by the bails on
  type_name/type_eq/has_impl and the bitcast guard.

`examples/170-pack-bare-value.sx` flips from the parse-error
lock-in to "0/1/3/4" — four call shapes of `len_of(..$args) ->
s64 { list := $args; return list.len; }`. The slice's `.len`
field carries the per-mono pack arity.

210/210 example tests + `zig build test` green.

The remaining 4A.bare slices (4 and 5) — resolveTypeArg
silent-arm fix for index_expr + smoke test of a real builder
walking $args — are separate commits per the cadence rule.

src/ast.zig

@@ -57,6 +57,7 @@ pub const Node = struct {
         many_pointer_type_expr: ManyPointerTypeExpr,
         optional_type_expr: OptionalTypeExpr,
         pack_index_type_expr: PackIndexTypeExpr,
+        comptime_pack_ref: ComptimePackRef,
         force_unwrap: ForceUnwrap,
         null_coalesce: NullCoalesce,
         deref_expr: DerefExpr,
@@ -374,6 +375,15 @@ pub const PackIndexTypeExpr = struct {
     index: u32,
 };
 
+/// `$<pack_name>` (no indexing) in expression position. Evaluates
+/// to a comptime `[]Type` slice — the WHOLE pack as data. Step 4
+/// final slice: lets builder fns walk the pack types and emit
+/// per-position code (the shape step 5's generic Into(Block) needs
+/// for its trampoline body).
+pub const ComptimePackRef = struct {
+    pack_name: []const u8,
+};
+
 pub const DeferStmt = struct {
     expr: *Node,
 };