Route session transactions through the persistence worker

[devmobasa]

Summary

Move session transaction orchestration into a shared production owner used by
both the Wayland overlay and session tests.

Route session disk operations through the persistence worker instead of
maintaining separate implementations in the runtime adapter and tests.

Changes

• Introduce a shared SessionTransaction context for input state, text
  measurement, session state, and persistence.
• Route Open, Save As, overwrite preflight, Clear, and saved tool-state resets
  through persistence operations.
• Keep save-before-open ordering, named-target validation, and overwrite checks.
• Commit target and canvas changes only after the required disk operations
  succeed.
• Preserve pending output-transition handling and session recovery rules.
• Reduce the Wayland session adapter to calls into the shared transaction owner.

Regression coverage

• Exercise the production transaction code with a real persistence worker.
• Retain coverage for target validation, overwrite handling, failed operations,
  saved data, and session transitions.
• Remove the duplicate test-side transaction implementation so tests follow
  the same orchestration path as the overlay.