[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.

Inizia da tests/test_open.py::test_load_schunk_returns_in_memory_copy e riproduci il problema con Pyodide 0.29.4, quindi esamina il percorso get_slice/__getitem__ di schunk.py e l’entry point blosc2_ext.pyx indicato nel traceback. Controlla la gestione della versione di Pyodide in .github/workflows/cibuildwheels.yml e wasm.yml. Il lavoro è completato quando lo slicing di SChunk caricati in memoria funziona su 0.29.4 senza introdurre regressioni negli ambienti 0.29.3 e 314.0.0, che superano i test.

Scritto dal modello di indicizzazione a partire dal testo della issue.