keeper: adopt the venue denial-classification seam from videre

[mfw78]

Problem. nullislabs/videre-nexum-module#11 ships a venue-supplied denial-classification seam: Venue::classify_denied (default Drop) plus a classifier-parameterised keeper path, so the generic sweep and the composable-cow sweep can derive the same RetryAction from one mapping. Shepherd still derives them independently: the composable-cow sweep calls cow_venue::classification::classify_denied directly and the generic path (stop-loss) drops every denial.

Scope.

• Bump the videre rev once keeper: let a venue supply denial classification to the generic retry path videre-nexum-module#11 merges.
• Override classify_denied on the CoW venue marker with cow_venue::classification::classify_denied.
• Derive the composable-cow sweep and stop-loss retry actions through the seam (with_classifier / retry_action::<V>), removing the direct call.
• Parity test: both paths return the same RetryAction for the same fault, including the InvalidEip1271Signature one-block grace.

Dependencies. nullislabs/videre-nexum-module#11 merged and revved.

AI Assistance: Claude Fable filed this follow-up from the videre#11 design (seam here, adoption in shepherd).