[WASM] SChunk.__getitem__/get_slice raises RuntimeError on Pyodide 0.29.4 (works on 0.29.3 and 314.0.0)

## Summary
On the WASM/Pyodide build, slicing an **in-memory** `SChunk` that was loaded from
a frame fails with `RuntimeError: Error while getting the slice` — but **only
under Pyodide 0.29.4**. The same code passes on Pyodide **0.29.3** (cp313) and
**314.0.0** (cp314).

The wheel tag is `pyemscripten_2025_0_wasm32` (ABI-level), so a wheel built/tested
against 0.29.3 still runs on a user's 0.29.4 runtime — meaning this is a genuine
runtime failure for 0.29.4 users, not just a CI artifact.

## Reproducer
`tests/test_open.py::test_load_schunk_returns_in_memory_copy`:

```python
urlpath = tmp_path / "schunk.b2frame"
data = np.arange(20, dtype=np.int32)
blosc2.SChunk(data=data, urlpath=urlpath, mode="w",
cparams={"typesize": data.dtype.itemsize})
loaded = blosc2.load(urlpath) # in-memory copy, urlpath is None
assert loaded[:] == data.tobytes() # <-- RuntimeError here
```

## Traceback

schunk.py:1114 __getitem__ -> get_slice(item.start, item.stop)
schunk.py:1058 get_slice -> super().get_slice(start, stop, out)
blosc2_ext.pyx:2058 -> RuntimeError: Error while getting the slice

## Environment matrix

```
│ Python / ABI │ Pyodide │ Result │
│ cp313 / pyemscripten_2025_0 │ 0.29.3 │ ✅ pass │
│ cp313 / pyemscripten_2025_0 │ 0.29.4 │ ❌ RuntimeError │
│ cp314 / pyemscripten_2026_0 │ 314.0.0 │ ✅ pass │
```

## Current workaround

.github/workflows/cibuildwheels.yml pins CIBW_PYODIDE_VERSION: 0.29.3 for the
cp313 wasm job (cibuildwheel 4.1's default is 0.29.4). This keeps CI green and
matches the wheels wasm.yml already ships, but does not fix the runtime for
0.29.4 users.

tests/test_open.py::test_load_schunk_returns_in_memory_copy から始めて、Pyodide 0.29.4 で失敗を再現し、その後 schunk.py の get_slice/__getitem__ パスと、traceback で示されている blosc2_ext.pyx のエントリポイントを調べます。.github/workflows/cibuildwheels.yml と wasm.yml での Pyodide バージョンの扱いを確認します。0.29.3 と 314.0.0 の、現在パスしている環境でリグレッションを発生させずに、メモリに読み込まれた SChunk のスライシングが 0.29.4 で動作すれば完了です。

索引モデルが issue の本文から書いたものです。