**FIXED** via the `xx <pack>` bridge (the preferred fix below), not by changing
the `..args` spread. `xx args` with a slice target now materializes the pack
into a runtime `[]Any`/`[]P` — see [examples/204-pack-xx-to-slice.sx](../examples/204-pack-xx-to-slice.sx).
`lowerXX`/the unary-op arm intercepts `xx <pack>` before the pack-as-value
check and calls the new `lowerPackToSlice` ([src/ir/lower.zig](../src/ir/lower.zig)).
The bare `..args` spread into a non-variadic `[]Any` param is still unsupported
(use `xx args`); left as-is.

# Symptom

Spreading a comptime pack `..$args` into a `[]Any` parameter — `f(..args)` where
`f` takes `items: []Any` — fails LLVM verification:

```
LLVM verification failed: Incorrect number of arguments passed to called function!
  %call = call i64 @log_count(ptr %0, { ptr, i64 }, { ptr, i64 }, double ...)
```

The spread passes the pack's N elements as N separate positional args instead of
materialising a single `[]Any` slice for the one `items` parameter.

# Reproduction

```sx
#import "modules/std.sx";
log_count :: (items: []Any) -> i64 { return items.len; }
forward :: (..$args) -> i64 { return log_count(..args); }
main :: () -> i32 { print("{}\n", forward(1, "hi", 2.5)); return 0; }
```

Expected: `3` (the pack spreads into the `[]Any` slice, like calling
`log_count(1, "hi", 2.5)` against a `[]Any` variadic would).

# Preferred fix — `xx args` (pack → slice materialization)

Rather than make the splat-y `..args` spread collapse into a single slice arg,
the cleaner spelling is an **`xx` cast**, which already means "erase/convert to
the expected type":

```sx
forward :: (..$args) -> i64 { return log_count(xx args); }   // target: []Any
```

`xx args` (target-typed) should materialize the pack into the expected slice:
- target `[]Any`  → box each pack element to `Any`, build `[N]Any` → `[]Any`;
- target `[]P`    → `xx`-erase each element to the protocol `P`, build `[N]P`
  → `[]P` (reuse the slice-of-protocol erasure landed in `packVariadicCallArgs`,
  issue 0052).

This reuses the existing `xx`/protocol machinery, reads naturally, and keeps
`..xs` reserved for true spreads into pack/variadic callees.

**Currently `xx args` errors** ("pack 'args' has no runtime value") because the
Step 2.7 pack-as-value check fires on the bare `args` operand before `xx` is
considered. The fix: in `xx` (unary_op `.xx`) lowering, intercept a pack operand
*before* the pack-as-value diagnostic and, when the target type is a slice,
materialize as above.

# Workaround today

Declare the forwarder as the **slice** variadic instead of a pack — then it's
already a runtime `[]Any` and forwards directly:

```sx
forward :: (..args: []Any) -> i64 { return log_count(args); }   // works -> 3
```

This is what `examples/162-pack-bare-args.sx` demonstrates.

# Verification

After the fix, `log_count(xx args)` (and the original `..args` form, if also
fixed) should print `3` and pass `sx ir` LLVM verification.
