M160: Extract editor panes into reusable controls

[Coasterpete]

Summary

Implements Milestone 160 by extracting the existing editor panes into reusable public Avalonia controls.

Changes

• Extracted Route, Viewport, Inspector, Math Plots, and Diagnostics into reusable controls.
• Preserved the existing grid layout, splitters, tabs, styling, events, selection synchronization, and radius-editing behavior.
• Reduced MainWindow to pane composition and cross-pane coordination.
• Added pane extraction contract tests.
• Added M160 implementation documentation.

Scope

This milestone makes no changes to:

• backend architecture
• authoring behavior
• sampling
• persistence
• engineering snapshot semantics

Validation

• 1,782 tests passed
• Solution builds with zero warnings and errors
• Editor startup smoke test passed
• git diff --check passed