Decision point logging for conditional routing

[JaimeStill]

Migrated from tailored-agentic-units/tau-orchestrate#2

Summary

Capture the reasoning and evaluation context at conditional routing points and edge transitions where the workflow makes a decision.

Requirements

• Record which predicate was evaluated and what it returned
• Capture the state snapshot at the moment of decision
• Log which route was selected and why (predicate name, result)
• Support audit trails for compliance-sensitive workflows

Design Constraints

• Backward compatible: existing Observer interface must not change; new capabilities extend through new observer implementations or optional interface methods
• No core package coupling: advanced observers should not add dependencies to the core orchestrate/observability package
• Event model stability: new event types can be added; existing ones should not change semantics

Acceptance Criteria

• Decision points produce audit-quality logs with state snapshots and predicate results
• All new observer implementations pass through the existing registry pattern
• No performance regression for existing NoOpObserver and SlogObserver usage