Vis.channels has no cache but .textures does — asymmetric

[gerchowl]

Tier 3 — consistency

The adversarial review flagged:

• `Vis.textures` is a property that fetches once and caches in
  `_textures` + `_fetched`.
• `Vis.channels` is a property that hits the client on every access
  with no cache:

@property
def channels(self) -> list[str]:
    if not self.has_mapping:
        return []
    return self.client.channels(self.source, self.material_id, self.tier)

`client.channels` is cheap — it reads the rowmap, which is in-memory on
the client after the first fetch — but still asymmetric with
`.textures`. Two rationales to pick from:

Options

(a) Cache channels too. Add `_channels: list[str] | None = field(...)`
and invalidate alongside `_textures` + `_fetched` in `setattr`.
Consistency.

(b) Document the asymmetry. Channels are cheap (rowmap lookup),
textures are expensive (HTTP range reads + PNG bytes). No need to
cache cheap.

Recommendation

(b) — cache-everything is a trap. The adversarial review's
observation is fair but the cost model is real: channels lookup is
O(1) against an already-cached rowmap on the client. Adding a second
cache field doubles the invalidation surface for almost no gain.

Minor action

Add one line to `channels` docstring noting "result comes from the
client's in-memory rowmap cache — cheap per-access, not cached on
this Vis."