Gap-7: Functions Diagnose (B → A) — Trigger/binding/Durable troubleshooting + tests

Parent: #1608

## Functions Diagnose: B → A

**Learning from:** Container Apps (B+) — has both reference docs AND integration tests for diagnostics

**Current state:** Has `functions/README.md` with ARG queries. Missing: integration tests, trigger-specific troubleshooting, binding error diagnosis.

## Deliverables

- [ ] Extend `references/functions/README.md` with:
- Trigger-specific troubleshooting (HTTP 5xx, timer misfires, queue poison messages, blob trigger delays)
- Binding error diagnosis (connection string issues, permission errors, SDK version conflicts)
- Durable Functions troubleshooting (stuck orchestrations, replay issues, task hub conflicts)
- Flex Consumption-specific issues (instance allocation, networking)
- [ ] Add 2 integration tests: trigger failure diagnosis, binding error troubleshooting

## Key Scenarios

- "My timer function is not firing" → NCRONTAB validation, singleton lock, scale controller logs
- "My queue trigger has poison messages" → dead-letter analysis, maxDequeueCount config
- "My Durable orchestration is stuck" → task hub status, purge instance history

> ⚠️ **Starting assessment** — domain experts should modify or contribute what they believe is essential.

Read references/functions/README.md alongside the existing functions/README.md to understand the current ARG queries and documentation structure. Identify the repository's integration-test entry point, then cover trigger-failure and binding-error diagnosis with the requested troubleshooting sections. Done means both README updates and two passing integration tests address the listed scenarios.

Written by the indexing model from the issue text.