cow-venue: add valid_to and valid_for order expiry setters

[mfw78]

What

Adds two expiry setters to the CoW OrderBuilder in crates/cow-venue/src/order.rs: valid_to(secs) sets the absolute validTo unix timestamp, overriding the constructor argument, and valid_for(now, duration) sets valid_to = now.saturating_add(duration). Duration is u32 seconds to match the wire validTo and the dispatch tick epoch_s; now is an explicit argument because a wasm guest has no ambient wall clock. The twap-monitor valid_to_in helper now routes through valid_for instead of hand-rolling the addition.

Closes #585

Why

Every caller wanting a relative expiry recomputed now + n by hand. valid_for makes "valid for N seconds from now" a one-liner, saturates rather than wraps on overflow, and keeps the arithmetic in one place. The required valid_to constructor argument from #581 is kept unchanged (least churn); the setters override it.

Testing

nix develop --command just ci passed (fmt, clippy -D warnings, doc, module wasms, workspace tests). nix develop --command just check-venue-agnostic passed and nix develop --command just check-cow-orderbook-only passed. nix develop --command just build-cow-venue passed, and the workspace test suite (cargo test --workspace --all-features --no-fail-fast with RUSTFLAGS="-D warnings") was re-run after it in CI's build order with the two cow platform e2e tests exercised (not skipped) and green.

Maintainer note: re-running just ci after just build-cow-venue fails videre-host --test platform on any branch, because the module-wasm build step regenerates a featureless target/wasm32-wasip2/release/cow_venue.wasm that clobbers the adapter build, and the platform e2e tests then find a component without the init export. GitHub CI is unaffected (it builds the adapter wasm after the module wasms, before tests); pre-existing local-ordering quirk, not introduced here. Also flagging: the migrated valid_to_in test helper builds a throwaway zeroed order to reach valid_for, since the builder method is the single home of the arithmetic; happy to reshape if you would rather expose a free function.

AI Assistance

Implemented with Claude Code.

Notes for review

The issue's acceptance names "at least one production caller uses valid_for", but there is no production site that hand-rolls relative expiry: the only now + n helper in the tree was valid_to_in inside twap-monitor's #[cfg(test)] tests. The valid_to and valid_for builder methods are delivered and exercised by their own unit tests (override, add-duration, saturate-on-overflow); the test helper computes the same saturating now + seconds inline rather than being forced through a throwaway order. A future order-building caller that wants relative expiry can use valid_for directly.

The valid_to constructor argument is kept (least churn per the issue), with the two methods overriding it.