New track for prior-session sepia tint:
- 3 new theme slots (prior_session_bg, prior_session_tint, prior_session_amount)
- per-palette state dict mirroring _brightness/_contrast/_gamma
- apply_prior_tint helper (float-only math per user requirement)
- 6 prior-session render sites wrapped (2 bubble_vendor swaps + 4 tint wraps)
- Theme Settings panel slider with persistence
Code-block tonemap fix is OUT OF SCOPE (upstream imgui_bundle 1.92.5
API only exposes 4-value PaletteId enum, no per-instance struct).
See spec §1.1.1 and design doc 'Honest constraint' section.
