Rich menu editor: motion, imagery, animated buttons, and disc-aware UI

[ScottMorris]

Tracks the rich menu editor programme: motion backgrounds, animated buttons, richer imagery, timeline editing, and disc-aware UI — with the design decisions that keep the same editor extensible to BDMV authoring.

Plan: docs/rich-menu-editor-plan.md (lands via #103).

Slice dependency graph

flowchart LR
    A["Slice A · format profiles\n& terminology"] --> B["Slice B · visual fidelity\n+ mirror retirement"]
    B --> C["Slice C · motion backend\n(MenuCompiler seam)"]
    C --> D["Slice D · timeline +\nanimation tracks"]
    D --> E["Slice E · video buttons"]
    B --> F["Slice F · themes &\ncomponents"]
    R["DCSQ player-compat\nresearch"] --> D

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The menu role model lands with whichever of Slice A / Slice C ships first. The colour-management spike and the lib-igs-author-plan.md revision are unordered.

Out of scope here

• The BD backend itself — docs/blu-ray-integration-plan.md phases 1–8.
• igs-author library work — tracked by the companion epic igs-author: BD menu authoring pipeline (IGS compile) libhdmv#1.