Fix Brush tool drawing into nested layers

[timon-schelling]

Drawing on a nested layer used to spawn a new brush layer because the tool only checked direct parent. Now it walks all ancestors.

Also removes hardcoded strokes input indices

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}