Clarify canvas runtime target detection

[chubes4]

Summary

• preserve canvases only when scripts directly request canvas context on the target selector
• omit decorative non-runtime canvases without noisy fallback diagnostics
• reduce broad canvas API false positives in runtime dependency parity

Testing

• composer test

AI assistance

• AI assistance: Yes
• Tool(s): openai/gpt-5.5 via OpenCode general subagents
• Used for: r14 canvas/runtime diagnostic analysis, implementation, and tests