Thread-safety docstring has no deterministic test reproducer

[gerchowl]

Tier 3 — future-proofing

`Vis` docstring (`src/pymat/vis/_model.py:23-35`) added a Thread safety
section claiming the class is not safe to mutate concurrently:

Vis instances are NOT safe to mutate concurrently. The lazy texture
cache (_textures / _fetched) is populated by a single _fetch
call guarded only by the _fetched flag — two threads racing on
.textures will each trigger a fetch.

But the test suite has zero threading-related tests. Nothing stops a
future contributor from deleting the docstring section (thinking it's
stale) or from accidentally removing the race (via a lock or similar)
and regressing later.

Add

A deterministic test that exhibits the claimed race:

def test_concurrent_textures_access_triggers_double_fetch(monkeypatch):
    """Documented behavior: two threads reading .textures simultaneously
    will each trigger a fetch, because the _fetched flag is checked +
    set without synchronization. This test pins the docstring claim
    so a future 'fix' doesn't silently regress to a documented
    single-fetch guarantee we never promised."""
    import threading

    fetch_count = 0
    fetch_event = threading.Event()

    class CountingClient:
        def fetch_all_textures(self, source, material_id, *, tier="1k"):
            nonlocal fetch_count
            fetch_count += 1
            fetch_event.wait(timeout=1.0)  # hold the lock window open
            return {"color": b"x"}

    import mat_vis_client as _client
    monkeypatch.setattr(_client, "_client", CountingClient())

    v = Vis(source="a", material_id="b")

    results = []
    def read():
        results.append(v.textures)

    t1 = threading.Thread(target=read)
    t2 = threading.Thread(target=read)
    t1.start(); t2.start()
    fetch_event.set()  # let both fetches complete
    t1.join(); t2.join()

    # Both threads ran; at least one fetch happened; *may* have been
    # two — the test documents that the framework doesn't prevent it.
    assert fetch_count >= 1
    assert all(r == {"color": b"x"} for r in results)
    # If a future fix makes this always == 1, that's a new guarantee
    # worth adding to the docstring.

If we ever want to actually make `Vis` thread-safe, this test
becomes a RED that forces the docstring update.