Add shared UI animation scheduler

[ProGraMajster]

Problem

The previous animation foundation used a Task.Delay(16) loop around a shared Stopwatch and mutable animation list. The continuation could leave the UI context, progress and lifecycle behavior were difficult to test deterministically, owner disposal was not part of the contract, and Windows/Android did not share an explicit application-lifecycle policy.

This PR replaces that foundation without implementing ThemeManager, Shape, navigation, or a full animated-layout system.

Architecture

• One lazily-created, process-wide scheduler for the framework's current single UI dispatcher.
• One shared, idle-aware pacing source; it starts for runnable work and stops after the final animation.
• Progress derives from monotonic Stopwatch.GetTimestamp time, not frame counts, so delayed frames catch up without extending duration.
• Timer requests are coalesced through the existing dispatcher; user updates and callbacks execute on the UI thread and never under the scheduler lock.
• Start, cancel, state reads, pause/resume, and shutdown are thread-safe.
• A manual clock, tick source, and dispatcher make tests deterministic without Thread.Sleep.

The design and alternatives are documented in:

• docs/architecture/ui-animation-scheduler.md
• docs/architecture/decisions/ADR-UI-Animation-Scheduler.md

Public API

The feature adds:

• AnimationScheduler
• AnimationHandle
• AnimationState
• AnimationOptions
• AnimationPolicy
• AnimationReplacementMode
• AnimationSchedulerDiagnostics
• IAnimationInterpolator<T>
• AnimationInterpolators

Existing FadeToAsync, TranslateToAsync, ScaleToAsync, RotateToAsync, and Switch behavior delegates to the new scheduler rather than maintaining a second timing system.

Cancellation and replacement

Animations are identified by owner reference plus string key. The default behavior replaces and cancels the matching animation; IgnoreNew keeps the existing handle. Cancellation is idempotent, never reports successful completion, removes the entry, and prevents later updates or repaint.

Control-owned work is canceled when the control is detached or disposed. Application shutdown permanently shuts down the default scheduler.

Easing and interpolation

Built-in linear/ease-in/ease-out/ease-in-out functions remain available and custom easing is supported. Finite overshoot is preserved for Back/Elastic-style functions; NaN, infinity, or exceptions fault only the affected animation and are reported through diagnostics/trace.

Typed interpolation covers float, double, int, PointF, SizeF, RectangleF, alpha-aware Color, Matrix3x2, GradientStop, and compatible brushes.

Brush integration

Solid, linear, radial, and sweep brushes support color, opacity, transform, geometry, gradient-stop, and spread-mode transitions.

• Local brush interpolation clones once and does not mutate either endpoint.
• Explicit AnimateTo helpers can intentionally mutate a shared observable brush or gradient stop.
• Incompatible brush types or gradient-stop structures fail before scheduling.
• Brush notifications request render invalidation only; layout-affecting control values continue to use layout invalidation.
• Replacing a dynamic resource detaches a control from an old brush that is still animating.

Lifecycle and reduced motion

The backend foundation now exposes a platform-neutral application lifecycle contract. Android's activity tracker reports foreground, background, and no-host states. Backgrounding pauses scheduler time and the tick source; foregrounding resumes remaining work without a time jump.

AnimationPolicy centrally supports animation enable/disable, reduced motion, and duration scaling. Disabled/reduced-motion and zero-duration work posts the final value once without starting the tick source. Design-time execution likewise settles at the final value instead of leaving an active timer.

Windows and Android impact

• Windows uses the existing UI dispatcher and application shutdown path. The main Windows target and Designer build successfully.
• Android uses the registered main-Looper dispatcher and activity lifecycle service. The backend and cross-platform sample compile for net10.0-android.
• Android support remains experimental. Physical-device frame pacing and lifecycle behavior still need manual verification.

Tests

Deterministic coverage includes:

• idle/start/stop behavior, elapsed-time progress, delays, dropped frames, and high animation counts;
• start/cancel during tick, idempotence, completion semantics, pause/resume, replacement, and shutdown;
• UI-thread dispatch and start from a background thread;
• duration scaling, disabled animations, reduced motion, easing boundaries/failures, and fault isolation;
• primitive, matrix, gradient-stop, and brush interpolation;
• dynamic-resource replacement, brush repaint, layout-versus-render invalidation, and no repaint after cancellation;
• control detach/dispose and Android lifecycle pause/resume.

Validation completed locally:

• restore: passed;
• Debug solution build: passed (0 errors);
• Release solution build: passed (0 errors);
• all solution tests: 595/595 passed;
• Windows target: passed with 0 warnings;
• Android backend target: passed with 0 warnings;
• Android sample target: passed with two existing XA0141 native page-size warnings;
• Designer, VSIX Debug, and VSIX Release: passed;
• ControlGallery and template/reference DemoApp Release builds: passed with 0 warnings;
• local pack: 8 version-1.8.0 nupkg files and 7 snupkg files, with README/icon/XML docs and Source Link payloads verified;
• feature diff: clean, no project/manifest/version changes.

ControlGallery

A dedicated opt-in page demonstrates opacity, color, easing, cancel/replace, gradient stops, brush transforms, parallel animations, reduced motion/disabled motion, and diagnostics. It starts no automatic work and cancels tracked handles/restores global policy when unloaded.

No screenshot or recording is attached; interactive Windows visual verification remains a manual review item.

Known limitations

• No repeat, auto-reverse, queue, blend, or animation groups yet.
• No general animated-layout abstraction; callers may animate layout-affecting values explicitly.
• Brush interpolation requires matching brush types and gradient-stop counts.
• The fallback pacing source is timer-based rather than synchronized to a native compositor frame callback.
• Native OS reduced-motion discovery is not implemented; applications set the central policy.
• Android physical-device performance, suspend/resume, configuration-change, and rendering behavior require manual validation.
• Existing helper calls with a negative integer duration now fail argument validation instead of being silently coerced to a minimal duration.

Next step

ThemeManager with JSON serialization and animated theme transitions, built on this scheduler. ThemeManager and Shape are deliberately not marked complete by this PR.