What is the Apigee Debug / Trace tool and how do you use it for troubleshooting?

<!-- Trace is started from the Apigee UI:
     Develop > API Proxies > [proxy name] > Debug tab

     Or via API:
     POST /organizations/{org}/environments/{env}/apis/{api}/revisions/{rev}/debugsessions

     A trace session captures:
     - Every policy that executed and its result
     - All flow variables and their values at each step
     - Request/response headers and body
     - Time spent at each step (ms)
     - Any faults raised and which FaultRule handled them

     Reading a trace:
     - Green step: policy executed successfully
     - Red step:   policy raised a fault
     - Grey step:  policy was skipped (condition was false)
     - Phase markers: ProxyRequest, TargetRequest, TargetResponse, ProxyResponse

     Common debugging workflows:
     1. Variable values wrong: check ExtractVariables step output
     2. Policy not executing: check condition expression in grey steps
     3. Auth failing: check VerifyAPIKey step - red with fault.name
     4. Slow response: check timing at TargetRequest/TargetResponse
     5. Wrong response body: check AssignMessage step output

     IMPORTANT: Trace sessions capture sensitive data (request bodies,
     headers, API keys). Limit trace in production environments. -->