fix(trader)(#30): short-TTL tarpit on proposal-timeout counterparty

[vrogojin]

Summary

Fixes #30. When a counterparty went silent during NP-0 negotiation, the matcher would deterministically re-pick the same dead candidate on every scan tick — burning the entire match cycle on a corpse while live candidates were starved. Now, after K=2 consecutive proposal failures, the counterparty is excluded from that intent's candidate pool for ~5 minutes.

What changed

• types.ts — new CancellationReason type; OnDealCancelled now receives a reason argument (transient — not persisted).
• negotiation-handler.ts — transitionDeal accepts an optional reason and forwards it to onDealCancelled. Each existing transitionDeal(_, 'CANCELLED', ...) callsite passes its semantic reason (proposal_timeout, proposal_send_failed, accept_send_failed, counterparty_rejected, sibling_cancelled, shutdown).
• intent-engine.ts — failedCounterparties value type promoted from Set<string> to Map<string, TarpitEntry> carrying failureCount, tarpitUntil, lastFailureAt. New recordCounterpartyFailure() (soft-threshold) and clearCounterpartyFailures() (success-reset). Existing markCounterpartyFailed() is now the strong-signal immediate-tarpit primitive (used by the W2 yield fall-through).
• trader-main.ts — onDealCancelled discriminates by reason: on proposal_timeout/proposal_send_failed, call recordCounterpartyFailure. On onDealAccepted (proposer side only), call clearCounterpartyFailures so consecutive really means consecutive.

Policy

Knob	Value	Why
Threshold (K)	2	Tolerate one lost Nostr DM without penalising peer
TTL	5 min	Skip ~10 scan cycles at default 30s interval; recovered peers rejoin quickly
Counter decay	10 min	A peer that flapped 2 hours ago doesn't get insta-tarpitted on one modern miss
Capacity per intent	1000 (LRU)	Inherited from existing bound

Test plan

• npm run lint — clean
• npm run typecheck — clean
• npm run test — 711/711 pass (10 new tests)
• npm run build — clean
• Adversarial steelman pass surfaced the success-reset hole; covered with clearCounterpartyFailures + dedicated test

Tests added

src/trader/intent-engine.test.ts:

• threshold gating (1 failure ≠ tarpit; 2 = tarpit)
• live candidate gets selected when dead one is tarpitted (issue's soak scenario)
• TTL expiry re-admits counterparty
• per-intent isolation (tarpit on A doesn't affect B)
• counter decays after 10 min of quiet
• accepts market_intent_id or local intent_id
• markCounterpartyFailed tarpits on first call
• clearCounterpartyFailures resets counter
• clearCounterpartyFailures on absent entry is no-op

src/trader/negotiation-handler.test.ts:

• forwards reason="proposal_timeout" on PROPOSED 30s timer
• forwards reason="counterparty_rejected" on incoming np.reject_deal
• forwards reason="shutdown" on cancelPending()