sdk: add the keeper macro and a typed venue client

[mfw78]

Add #[videre::keeper], the keeper-author mirror of the venue macro, letting authors drive a venue through a typed VenueClient<V> instead of hand-writing list<u8> marshalling.

Why

The venue author gets #[videre::venue]; the keeper author has no equivalent and must hand-write byte marshalling to drive a venue over videre:venue/client. A macro plus a typed, alloy-style client closes that gap and keeps the hot path free of boxing. Part of milestone M3: Videre SDK, macros and DX. Blocked by: #[videre::venue]: single blessed authoring path emitting impl VenueAdapter, videre-quote. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Add #[videre::keeper]: the author writes logic against a typed VenueClient<V>, and the macro wires the event subscriptions and the videre:venue/client import.
• Provide VenueClient<V> as an alloy-style typed wrapper exposing quote, submit, status and cancel, with typed bodies via the venue's IntentBody, keyed by VenueId.
• Use native AFIT for the hot traits under MSRV 1.94 so there is zero boxing on the hot path.
• Prove the path by driving echo-venue from a keeper.

Done when

• #[videre::keeper] emits a worker that drives a venue via a typed VenueClient<V> (alloy-style, typed rather than list<u8>) wrapping videre:venue/client and wiring the event subscriptions.
• A keeper written against VenueClient<CowVenue> compiles and calls quote, submit, status and cancel with typed bodies.
• Dispatch is static via native AFIT, with zero boxing on the hot path.

[mfw78]

Landed via #460 (squash-merged into dev/m1 as 41051fa). Closing manually since the merge target is not the default branch.