fix(inference): clamp dense walk top-k to feature count

[citizenu03bb]

Title:
fix(inference): clamp dense walk top-k to feature count

Summary:

• Clamp unlimited dense WalkFfn top-k to the layer feature count.
• Add a regression test covering WalkFfn::new_unlimited.

Why:
WalkFfn::new_unlimited previously returned usize::MAX from top_k_for
when no explicit per-layer k was configured. Dense paths that used this value
as a selection bound could request more features than exist. The unlimited
dense case should mean "all features in this layer", not an unbounded sentinel
escaping into execution.

Verification:

• cargo test -p larql-inference dense_top_k_for_clamps_to_layer_feature_count --lib
• cargo test -p larql-inference --lib

Branch:
pr/walk-ffn-dense-topk-clamp

Commit:
d75cd5d