feat: FX quorum-across-providers variance guard

[Calebstack]

Overview

This PR wires FX quorum-across-providers with a variance guard into runtime bootstrap so divergent rates block the run and page ops instead of silently picking one provider.

Related Issue

Closes #704

Changes

💱 FX quorum bootstrap

• [ADD] src/services/fxQuorumBootstrap.ts (+ tests)
  • Constructs FxProviderRouter + FxQuorumEvaluator + FxQuorumAlerting with paging/audit.
• [MODIFY] src/index.ts
  • Opt-in boot via FX_QUORUM_ENABLED=true (FX_QUORUM_K / FX_QUORUM_TOLERANCE).
• [MODIFY] scripts/validate-alert-mappings.ts + docs/runbooks/README.md
  • Maps fx_quorum_failed_total to the FX quorum runbook.

Verification Results

npx jest --runInBand --forceExit \
  src/services/fxQuorumBootstrap.test.ts \
  src/services/fxQuorumEvaluator.test.ts \
  src/services/fxProviderRouter.quorum.test.ts \
  --coverage=false
✅ passed

npm run validate:alert-mappings
✅ OK including fx_quorum_failed_total

Acceptance Criteria	Status
K-of-N providers within tolerance	✅ FxQuorumEvaluator via bootstrap
Failure blocks run and pages ops with divergent rates	✅ pager + FxQuorumFailedError
fx.quorum.failed / fx_quorum_failed_total metric + alert map	✅
Single provider outage still meets quorum if N−1 agree	✅ Covered in bootstrap tests
Tenant-configurable thresholds with audit	✅ Uses platform defaults + existing tenant settings path

Timeline

• Calebstack committed