refactor(twap-monitor): port to Host trait + MockHost tests (BLEU-854)

[brunota20]

Summary

Closes the M2-side gap that surfaced in the post-M3 QA: twap-monitor still called `chain::request`, `local_store::*`, `cow_api::submit_order` directly inside dispatch logic, and the 13 existing tests covered only parsers and encoders. The state machine (indexer / poll / submit / retry / lifecycle) was unverified in unit.

Same recipe as #22 (price-alert / BLEU-851) and #23 (stop-loss / BLEU-852):

• `strategy.rs` (new) - pure logic against `shepherd_sdk::host::Host`. `LogView<'a>` and `BlockInfo` keep the strategy wit-independent; `on_logs` / `on_block` are the entry points.
• `lib.rs` (rewritten, 665 -> 165 lines) - wit-bindgen `generate!`, `WitBindgenHost` adapter implementing all four SDK host traits, `Guest` impl that destructures `types::Event` and delegates to `strategy`.

Tests

Status	Test
Preserved	13 pure tests (decoders, encoders, watch-key round-trips, BLEU-830 outcome mapping)
New (MockHost)	`index_records_new_watch_on_conditional_order_created`
New (MockHost)	`index_overwrites_in_place_on_redelivered_log` (re-org replay guard)
New (MockHost)	`poll_skips_when_next_block_gate_is_in_future`
New (MockHost)	`poll_ready_submits_order_and_persists_submitted_uid`
New (MockHost)	`submit_transient_error_leaves_state_unchanged_for_next_block`
New (MockHost)	`submit_permanent_error_drops_watch`
New (MockHost)	`poll_dont_try_again_drops_watch_and_gates` (uses a real `OrderNotValid` selector via the SDK-exported `IConditionalOrder` sol! interface)

Total: 20 tests (was 13).

Numbers

`.wasm` (release wasm32-wasip2)	313,926 bytes
Em-dashes added	0

Linear: BLEU-854.

Parallel to BLEU-855 (ethflow-watcher), which lands next stacked on this branch.

Test plan

• `cargo test -p twap-monitor` -> 20 passing.
• `cargo clippy --all-targets --workspace -- -D warnings` clean.
• `cargo build --target wasm32-wasip2 --release -p twap-monitor` clean.
• `grep -rn '—' modules/twap-monitor/` -> 0 matches.

[linear-code]

BLEU-854