cow: bump the cow-rs rev and retire composable.rs upstream

[mfw78]

Unstacked tail of the convergence train; completes #398 done-when item 2.

Part of #398's done-when lives in cow-rs, not shepherd: composable.rs and the stale registry constant are upstream. Delete them there and bump shepherd's workspace patch rev from 17fc0c5f.

Why

Leaving the legacy poll surface in cow-rs invites reuse and keeps a dead constant that shepherd's own grep gate cannot see. #398's scope names the file, but the deletion physically happens upstream, so it needs its own coordinated change and rev bump.

Scope

• In nullislabs/cow-rs: delete composable.rs and the stale COMPOSABLE_COW constant (mirror issue to be filed there; this issue tracks the shepherd side).
• Bump the workspace patch rev; shepherd builds and tests green on the new rev.

Done when

• cow-rs contains no composable poll surface.
• shepherd pins the new rev and CI is green.
• cow: swap the composable-cow poll wire and delete the legacy adapter #398 done-when item 2 is fully satisfied.

Blocked by: #652.