feat(twap): precise polling hints for watch towers

[brunota20]

What

TWAP.getTradeableOrder now reverts with the precise polling errors from IConditionalOrder instead of the generic OrderNotValid("not within span") (and the transitive OrderNotValid("before twap start") / "after twap finish" raised inside TWAPOrderMathLib.calculateValidTo):

• Pre-t0 (before the first part starts): PollTryAtEpoch(t0, "before first part")
• Post total span (every part settled / passed): PollNever("all parts settled")
• Between parts of an in-progress TWAP (current part's span elapsed but next part not started): PollTryAtEpoch(nextPartStart, "between parts")

validate() is invoked up-front so invalid bundle configurations still revert with their original OrderNotValid(INVALID_*) reasons (no taxonomy change there). OrderNotValid(NOT_WITHIN_SPAN) is kept as a defensive fallback (should be unreachable).

Why

The PollTryAtEpoch / PollNever errors exist in IConditionalOrder specifically for watch towers to optimise polling, but TWAP.sol never used them. Watch towers (CoW DAO WatchTower + our Shepherd implementation) currently have to either poll every block (wasteful) or build out-of-band logic to know when the next part starts (e.g. our COW-1077 _twap_calldata.py helper that pins t0 = now-60).

With the precise hints, watch towers stop polling between parts entirely and remove finished TWAPs from their watch set without trial-and-error.

Backward compatibility

• PollTryAtEpoch and PollNever are pre-existing errors already on the IConditionalOrder interface — no interface change.
• Watch towers that handle the full error taxonomy (per the interface spec) gain efficiency immediately.
• Watch towers that only catch OrderNotValid continue to function: they will see an unknown revert and treat it as "skip this watch this block", same defensive behavior as today (no order is silently surfaced as valid).
• No selector changes to existing functions; no public-symbol removal.
• The existing in-repo tests test_getTradeableOrder_FuzzRevertIfBeforeStart, _FuzzRevertIfExpired, _FuzzRevertIfOutsideSpan, the cabinet-based variants, and the test_simulate_fuzz catch block were updated to expect the new (more informative) errors — these are internal regression tests, not downstream consumers.

Tests

4 new tests in ComposableCoW.twap.t.sol covering the three new branches plus a regression for the in-window case:

• test_twap_reverts_with_PollTryAtEpoch_before_t0
• test_twap_reverts_with_PollNever_after_total_span
• test_twap_reverts_with_PollTryAtEpoch_between_parts
• test_twap_existing_within_span_behavior_unchanged

Full suite: 78/78 pass (excluding the pre-existing _invariant and test_simulate_fuzz flake — the latter is an OOG counterexample present on main before this PR, unrelated to polling-hint changes; passes with reduced fuzz runs).

Closes

Addresses the "enhanced polling interfaces" for the TWAP handler.