Add a troubleshooting table to the custom adapter guide

[myheart521]

Goal

Help first-time adapter authors diagnose common integration failures without reading evaluator internals.

Acceptance criteria

• Document wrong horizon, NaN or infinite state, mismatched context ID, mismatched action name, and action-ignoring prediction symptoms.
• For each symptom, name the likely adapter-boundary cause and one bounded check.
• Include commands for the starter smoke and focused test module.
• Explain that zero Top-1 Regret from the linear starter is a wiring check, not a research result.
• Keep English instructions clear and add a short Chinese troubleshooting summary.
• Verify mkdocs build --strict and repository-local links.

Starting point

Edit docs/adapters/CUSTOM_ADAPTER.md after the contributor-readiness PR is merged.