M167: Interactive authoring foundation

[Coasterpete]

Summary

This PR completes M167, establishing the first interactive authoring foundation for Quantum.

The milestone introduces frontend-neutral transactional editing, live preview infrastructure, latest-wins evaluation scheduling, and the first production live editing workflow while preserving Track Layout Package V2 compatibility.

Included milestones

M167.1 — Measurement and redundant compilation cleanup

• Added scoped authoring pipeline instrumentation.
• Eliminated redundant graph compilation throughout editor workflows.
• Bound compile results to immutable source graphs.
• Introduced prepared graph states containing:
  • source graph
  • compile result
  • canonical package JSON
• Reduced compiler invocations:
  • ApplyGraphEdit: 5 → 1
  • ReplaceGraph: 2 → 1
  • Save: 2 → 0
  • Open: 3 → 1
  • Undo/Redo: 2 → 0

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M167.2 — Frontend-neutral authoring session

Added Quantum.Application.

Implemented:

• TrackAuthoringSession
• structural revision identities
• transactional editing
• provisional evaluation
• exact candidate adoption
• unified dirty-state ownership
• prepared-state history
• zero-compilation commit/undo/redo

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M167.3 — Latest-wins evaluation scheduling

Implemented:

• serialized one-running / one-latest-pending scheduler
• structural stale-result rejection
• throttling
• cancellation hints
• fault reporting
• scheduler metrics
• synchronous and opt-in background execution

Commit continues to reuse the exact accepted candidate with zero recompilation.

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M167.4 — First live interactive edit

Implemented the first complete live editing workflow.

Features include:

• live straight-length scrubber
• provisional preview
• session-owned publication
• exact asynchronous commit
• invalid preview handling
• camera preservation
• station preservation
• stable-node selection
• command conflict handling
• real Avalonia headless interaction tests

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M167.5 — Live-edit hardening

Completed:

• authoritative session-owned history
• lifecycle-safe shutdown/replacement
• stale/post-lifecycle diagnostics
• keyboard accessibility
• configurable scrub sensitivity
• Release performance measurements
• percentile latency reporting
• scheduler validation

Validation

• 1,948 Release tests passed
• 0 failed
• 0 skipped

Working tree clean.

Compatibility

• Track Layout Package V2 preserved.
• Existing one-shot editing remains supported.
• Live editing currently targets StraightSectionDefinition.Length as the reference interaction.
• Background scheduling remains opt-in pending broader thread-safety validation.

Follow-up

The next milestone (M168) will focus on improving the live authoring experience, including reliable live preview during section creation and continued evolution toward an FVD-style authoring workflow.