feat(failsafe): document priority axioms, add conflict resolution tests, and A3 fallback notification

[Rpirayesh]

Summary

The failsafe priority framework applies five axioms (A1–A5) to resolve conflicting failure conditions, but these rules were undocumented and untested. When the framework silently degraded from RTL to Land (e.g., GPS lost mid-RTL), operators received no notification.

This PR addresses all three gaps:

• Documentation — inline comments with worked examples for each axiom in failsafe.cpp and framework.cpp
• Unit tests — 4 new GoogleTest cases covering the highest-risk conflict scenarios
• Operator notification — notifyA3Fallback() fires a mavlink_log_critical + structured event when the framework degrades an action, with an anti-spam guard (fires at most once per worsening step)
• Log validator — validate_a3_fallback_log.py verifies A3 fallback event counts from ULogs

Files Changed

File	Change
failsafe.cpp	Priority axiom comments with worked examples (navigator_failure, geofence, battery)
failsafe_test.cpp	4 new tests + FailsafeTesterWithFallbackCapture fixture
framework.cpp	A3 fallback event emission, competing-conditions diagnostic
framework.h	notifyA3Fallback() virtual method
validate_a3_fallback_log.py	Post-flight ULog validator for A3 fallback chain

New Tests

Test	Axiom
conflict_higher_priority_wins	A1 — highest severity dominates
conflict_geofence_breach_during_rtl	A4 — duplicate RTL downgrades to Warn
rtl_fallback_to_land_when_home_invalid	A3 — RTL→Land when home unavailable
land_overrides_active_rtl_without_delay	A1 + clearDelayIfNeeded

Testing

make tests TESTFILTER=failsafe