has_mapping should include tier or tier should never be None

[gerchowl]

Tier 3 — narrow edge, minor

`Vis.has_mapping` checks only `source` and `material_id`:

@property
def has_mapping(self) -> bool:
    return self.source is not None and self.material_id is not None

But `tier` is also part of the identity triple. If someone sets
`vis.tier = None` explicitly (bypassing the default `"1k"`),
`has_mapping` still returns True but `client.fetch_all_textures(src, id,
tier=None)` will blow up downstream with an unhelpful error.

Options

(a) Gate in `has_mapping`: add `and self.tier is not None`. Narrows
the bad-state surface.

(b) Make tier required: change the dataclass field to `tier: str`
(no `None`) with a non-None default. A `vis.tier = None` assignment
then fails at assignment time via a `setattr` guard.

(c) Leave as-is: the surface is very narrow (explicit `None` only;
default `"1k"`), and we have `Tier 2/#63` round-trip coverage that
would flush it.

Recommendation

(a) is lowest-cost and most correct. (b) over-constrains for
the rare future where `tier=None` might mean "client default."

Test

def test_has_mapping_requires_tier(self):
    v = Vis(source="a", material_id="b")
    assert v.has_mapping  # default tier="1k"
    v.tier = None
    assert not v.has_mapping  # explicitly None → no mapping