fix: preserve FIXP recovery and expose journal sizing metrics

[pedrosakuma]

Summary

• document and regression-test graceful FIXP shutdown recovery
• expose durable journal append byte/frame counters for sizing
• document production persistence behavior and cold-journal alerts

Issues

• Closes Confirm whether graceful SIGTERM shutdown preserves FIXP session state/journal #613
• Relates to Confirm production/staging overlays enable retransmitPersistenceDir #614
• Relates to Validate cold-journal retention sizing (256MiB/24h) under production throughput #615

Validation

• dotnet restore SbeB3Exchange.slnx
• dotnet build SbeB3Exchange.slnx --no-restore -c Release
• dotnet format SbeB3Exchange.slnx --verify-no-changes --no-restore --severity warn
• targeted shutdown, journal, and metrics tests
• full Release test suite passed on rerun; one prior full-suite run had an isolated Host.Tests parallelism flake in GtRestatementE2ETests, which passed in isolation, serial mode, and subsequent full runs.

Sizing defaults remain unchanged pending production throughput data.

[pedrosakuma]

Code review obrigatório concluído no diff completo do PR: nenhum problema de alta confiança encontrado. O PR permanece draft até os checks remotos concluírem.