Material(vis={...}) loops setattr, causing redundant cache invalidation per identity field

[gerchowl]

Tier 3 — cosmetic perf

`src/pymat/core.py:71-75` (and the mirror at `:591-593`) applies the
`vis={...}` constructor kwarg by looping:

if vis:
    for key, value in vis.items():
        setattr(mat.vis, key, value)

If the user passes `vis={"source": "ambientcg", "material_id": "Metal012"}`,
each `setattr` trips `Vis.setattr` — invalidating a cache that's
already empty (so no user-visible bug), but performing redundant work.

Worse: if the dict iteration order happens to set `source` first and
`material_id` second, there's a brief intermediate state where
`has_mapping` returns False (`material_id is None`). Only observable
under threading or reentrant code, but it's a representable
inconsistency.

Fix

Batch the identity updates. Either:

(a) Apply non-identity scalars first, then identity atomically:

if vis:
    # Split identity keys from scalars
    identity = {k: vis[k] for k in ("source", "material_id", "tier") if k in vis}
    scalars = {k: v for k, v in vis.items() if k not in ("source", "material_id", "tier")}

    for key, value in scalars.items():
        setattr(mat.vis, key, value)

    # Then set identity via a dedicated method that invalidates once
    if identity:
        mat.vis._set_identity(**identity)  # new method — see below

(b) Add `Vis._set_identity(source=None, material_id=None, tier=None)`:

def _set_identity(self, *, source=None, material_id=None, tier=None):
    """Atomic identity update — all three fields in one invalidation."""
    if source is not None: super().__setattr__("source", source)
    if material_id is not None: super().__setattr__("material_id", material_id)
    if tier is not None: super().__setattr__("tier", tier)
    # Single invalidation regardless of how many fields changed
    super().__setattr__("_textures", {})
    super().__setattr__("_fetched", False)

Test

def test_atomic_identity_update_via_material_kwarg(self):
    m = Material(name="test", vis={"source": "a", "material_id": "b"})
    # has_mapping must be True after construction — never False
    # in an intermediate state
    assert m.vis.source == "a"
    assert m.vis.material_id == "b"
    assert m.vis.has_mapping

    # Verify single invalidation: prefill cache, then re-apply kwarg
    m.vis._textures = {"color": b"x"}
    m.vis._fetched = True
    m.vis._set_identity(source="c", material_id="d")
    assert m.vis._textures == {}
    # Confirm atomic: no race-window where only one was updated

Low priority. Ship if (b) gets picked up; skip if short-circuit from #64
lands first (that alone makes the redundant clears free).