ffi M5.A.next.4.3: $args[$i] in expression position — source construction

Final slice of the .type_tag activation. Sx code can now
construct Type values through the `$<pack>[<int_literal>]`
syntax in expression position. Lowering emits the new
`const_type(TypeId)` opcode; the interp materialises
`Value.type_tag(TypeId)`; reflection intrinsics + cmp_eq
read it kind-honestly.

Plumbing:

- src/parser.zig: `parsePrimary` accepts `$<ident>[<int_literal>]`
  at the front of every expression. Emits a `pack_index_type_expr`
  AST node — same node already used in TYPE positions in step 3,
  now extended to expression positions.

- src/ir/lower.zig: two places teach the new node.
  - `lowerExpr` arm: looks up `pack_arg_types[name][index]`, emits
    `builder.constType(arg_tys[index])`. OOB / no-binding paths
    emit a focused diagnostic + a `constType(.void)` placeholder
    (loud failure preserves silent-error budget).
  - `resolveTypeArg` arm: the same lookup, but returns the
    TypeId directly. Used by the lower-time fast paths in
    `tryLowerReflectionCall` + `tryConstBoolCondition` so
    `type_name($args[0])`, `type_eq($args[0], s64)`, and
    `has_impl(...)` all see the bound TypeId rather than
    falling through to the `.s64` default that the silent-arm
    rule forbids.

The two arms ensure both runtime AND compile-time paths use
the same source-of-truth (`pack_arg_types`), so per-mono
dispatch via `inline if type_eq($args[0], s64) { ... }` folds
at compile time as expected.

`examples/169-pack-value-dispatch.sx` exercises both shapes:
- `type_name($args[0])` returns the per-mono concrete type
  name ("s64", "string", "f64").
- `inline if type_eq($args[0], s64) { ... }` ladder dispatches
  per-mono ("got s64", "got string", "got bool", "got other").

209/209 example tests + `zig build test` green.

What's now possible end-to-end:

  show :: (..$args) -> string => type_name($args[0]);
  show(42)    // "s64"
  show("hi")  // "string"

  describe :: (..$args) -> string {
      inline if type_eq($args[0], s64) { return "got s64"; }
      ...
  }

The "by the book" activation is complete:
- foundation (const_type opcode, interp variant, helpers) — 4.0
- interp reflection arms (type_name / type_eq / has_impl) — 4.1
- box_any/display audit + bitcast guard — 4.2
- source-language construction via $args[$i] — 4.3

Step 5 (generic Into(Block) impl in stdlib) is now fully
unblocked — its trampoline body can interpolate per-mono types
both in type positions AND in expression positions.

src/parser.zig

@@ -2293,6 +2293,40 @@ pub const Parser = struct {
 
     fn parsePrimary(self: *Parser) anyerror!*Node {
         const start = self.current.loc.start;
+        // Pack-index in expression position: `$<pack_name>[<int_literal>]`.
+        // Yields a `pack_index_type_expr` AST node (same shape used in
+        // type positions in step 3) — lowering resolves it to a
+        // `const_type(TypeId)` value at the bound pack-arg type. Step 3
+        // already handles the same node in type positions; this arm
+        // extends it to value expressions, completing the round trip
+        // for builders that pass Type values to `type_name`/`type_eq`
+        // /etc at interp time.
+        if (self.current.tag == .dollar) {
+            self.advance();
+            if (self.current.tag != .identifier) {
+                return self.fail("expected pack name after '$'");
+            }
+            const pname = self.tokenSlice(self.current);
+            self.advance();
+            if (self.current.tag != .l_bracket) {
+                return self.fail("expected '[' after '$<pack_name>' (pack indexing requires a literal index in expression position)");
+            }
+            self.advance(); // skip '['
+            if (self.current.tag != .int_literal) {
+                return self.fail("expected integer literal in pack index");
+            }
+            const idx_text = self.tokenSlice(self.current);
+            const idx_val = std.fmt.parseInt(i64, idx_text, 10) catch {
+                return self.fail("invalid integer literal in pack index");
+            };
+            if (idx_val < 0) return self.fail("pack index cannot be negative");
+            self.advance();
+            try self.expect(.r_bracket);
+            return try self.createNode(start, .{ .pack_index_type_expr = .{
+                .pack_name = pname,
+                .index = @intCast(idx_val),
+            } });
+        }
         switch (self.current.tag) {
             .int_literal => {
                 const text = self.tokenSlice(self.current);