fix: Opinion BUY cap units + ungated fallback + double breaker record

[Frank-zhu0404]

Closes #127

Summary

Follow-up to #115 / #120. Fixes three remaining safety issues around Opinion / batch execution:

1. Opinion BUY maxOrderSize uses wrong units — Opinion BUY size/amount is quote-token spend (USDC), not shares. Validation and breaker accounting now use size for Opinion BUY (still price * size for SELL / other venues).
2. Ungated Opinion place fallback — opinion_place_order / opinion_place_orders_batch no longer call the venue directly when executionService is absent; they require the execution service so breaker, maxOrderSize, and dry-run apply.
3. Fallback batch orders double-recorded to the circuit breaker — the per-order fallback path now uses executeOrder instead of buyLimit/sellLimit, so the final batch loop records each result once.

Test plan

• tsc --noEmit clean
• Extended tests/execution/batch-order-safety.test.ts:
  • Opinion BUY cap treats size as quote spend (price 0.10 / size 1000 rejected at $100 cap; size 100 allowed; SELL still uses price×size)
  • Unknown-venue fallback records exactly one breaker trade
• Existing batch dry-run / size / breaker cases still pass