feat(execution): 체결가 0/None 시 지정가 백필 가드 + WARN 로깅 (dcy-032/BAT-048)

REQ-095(슬롯 평단·원장 price 지정가 백필, 0 오염 방지) + REQ-096(백필/미해결 WARN 로깅) 구현.
지정가 부재 시 skip+강WARN. application 단일화(_resolve_fill_price), wire/LedgerTrade 스키마 불변.
575 passed/0 fail, opus QC PASS. REQ-097 알람=스파이크 산출물만(impl 후속).

- _resolve_fill_price 헬퍼 추가: avg_price 검증, limit_price 백필, 미해결 WARN
- _on_filled/_on_partial_cancel 통합: 백필 적용, 슬롯/원장 일관성
- _record_ledger 시그니처: resolved_price 파라미터 (backward compat)
- test_execution_fill_price_backfill.py: 21 시나리오 (avg=None/0, limit=있음/없음 등)
- docs: dcy-032, req-095/096/097, bat-048 설계/검증

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VHtM5Ws1HKCKQ4JGi6GRb5

docs/batches/index.md

@@ -55,3 +55,4 @@
 | BAT-045 | standard | released | REQ-084, REQ-085, REQ-087 | dcy-029 (issue #29, P0) | [bat-045](./bat-045_20260616_observability-logging-foundation/) |
 | BAT-046 | standard | released | REQ-086, REQ-088, REQ-089, REQ-090 | dcy-029 (issue #29, P1) | [bat-046](./bat-046_20260616_observability-classification-pnl-runbook/) |
 | BAT-047 | standard | release-candidate | REQ-091, REQ-092, REQ-093, REQ-094 | dcy-031 | [bat-047](./bat-047_20260619_event-publisher-connection-reliability/) |
+| BAT-048 | standard | in-delivery | REQ-095, REQ-096, REQ-097 | dcy-032 | [bat-048](./bat-048_20260621_fill-price-backfill-guard-logging-alert-spike/) |