TUI - Program-scoped animation runtime

[krissetto]

First of many TUI improvement proposals :)

This PR sets some animation-related foundations for enabling a better overall TUI UX where we can:

• centralize spinners, durations, transitions, effects, etc for easier re-use
• make time based animations and effects, so frame rate changes or system load doesn't affect how long animations run
• start introducing more advanced effects more easily and robustly, e.g. fades, transitions, etc
• improve TUI re-rendering logic to reduce cpu usage
• facilitate integration of docker agent components into other pre-existing TUIs

Why

The current global coordinator advances animations by frame count and does not own queued ticks. Under event-loop delays, animations slow down; stale or duplicate tick chains also cannot be rejected cleanly.

What changed

• Adds one program-owned animation runtime with a single shared tick lease.
• Rejects stale, duplicate, and foreign ticks; stopping invalidates queued work.
• Drives transitions and spinners from elapsed duration, so missed frames do not change animation speed.
• Preserves the package facade during partial migration: delivered legacy ticks re-arm without Runtime.Accept, and IsCurrentGen remains a side-effect-free generation check. This matches some consumer's direct use of TickMsg, StartTick*, HasActive, and IsCurrentGen.

Component adoption of the new infra is for later PRs. The legacy facade can be removed after components and consumers have migrated.

flowchart LR
    Root["TUI root"] --> AnimationRuntime["Program animation runtime"]
    AnimationRuntime -->|shared tick| Components
    Components -->|subscribe while active| AnimationRuntime

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flowchart LR
    Delivery["Tick delivery"] --> Clock["Elapsed time"]
    Clock --> Transition["progress = elapsed / duration"]
    Clock --> Spinner["frame = elapsed / cadence"]

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[aheritier]

👋 This PR has merge conflicts with the base branch. Please rebase or merge the latest base branch and resolve them. I've moved it to draft and added status/needs-rebase; it'll be picked back up automatically once the conflicts are cleared.