cow: swap the composable-cow poll wire and delete the legacy adapter

[mfw78]

Delete composable.rs and LegacyRevertAdapter and switch the composable-cow poll onto the fork's structured non-reverting getTradeableOrderWithSignature. This is gated on the ComposableCoW fork being deployed and must not be coupled to the keeper port.

Why

The structured non-reverting poll cannot be instantiated until the ComposableCoW fork is deployed: Verdict::Post is the one variant LegacyRevertAdapter never produces, and Verdict::NeedsInput is dead surface until IOrderModule and the fork land. This wire-swap is hard-blocked on the fork's deployments/networks.json being non-empty on a shepherd target chain. It must stay decoupled from the keeper port, otherwise the whole shepherd split freezes behind a third-party deployment clock. Part of milestone M4: CoW on the generic seam (the shepherd bundle). Blocked by: cow-onvidere-epic; composable-cow-keeper-port. See docs/design/videre-split-plan.md and docs/design/issue-milestone-plan.md.

Scope

• Delete composable.rs and LegacyRevertAdapter.
• Switch the poll onto the fork's structured non-reverting getTradeableOrderWithSignature.
• Fully populate Verdict::Post; wire and test the Verdict::NeedsInput arm.
• Model next_poll_timestamp as an Option or NextPoll rather than the 0 sentinel that collides with the fork wire.
• Delete the legacy host-extension surface in wit/shepherd-cow.

Done when

• The work proceeds only once the fork's deployments/networks.json is non-empty on a shepherd target chain.
• composable.rs and LegacyRevertAdapter are deleted.
• Verdict::Post is fully populated by the structured non-reverting poll.
• The Verdict::NeedsInput arm is wired and dispatch-tested.
• Verdict::Post.next_poll_timestamp is modelled as Option or NextPoll, not the 0 sentinel that collides with the fork wire.
• The legacy host-extension surface in wit/shepherd-cow is deleted.

[mfw78]

The one arm of this issue that is not gated on the fork deployment, modelling Verdict::Post::next_poll_timestamp as Option<u64>, is detached to #567 and rides with the #538 pass. The remaining wire swap and LegacyRevertAdapter deletion stay here, labelled blocked, until the nullisLabs fork is deployed somewhere. Tracked adjacent to the M4 follow-up epic.

[mfw78]

Re-homed from M4 to M8. This is not cleave-dependent: it stays in the shepherd (L3) repo after the carve and is implemented post-cleave, whenever the ComposableCoW fork deploys. Keeping it out of M5/M6/M7 so no cleave milestone's completion is coupled to a third-party deployment clock (as this issue's own rationale warns).