feat(widget): add runtime window visibility control

[doodlewind]

Summary

• add a coalesced runtime Show/Hide request to both 3D and flat widget products
• keep fixed ticks alive while a native widget window is hidden
• suppress presentation while hidden and arm one fresh frame when the window is shown again
• cover visibility coalescing and the hidden-to-shown repaint latch with focused unit tests

Scope

This PR intentionally contains only the reusable pocket-widget capability needed by long-lived tray/MCP-controlled products. Persona-specific rendering, catalog, bridge, MCP, acceptance tooling, benchmark code, and documentation live in the companion Pocket Character Draft PR:

• feat(persona): add Pocket-native parity POC pocket-character#3

The earlier Persona vertical slice was moved out of this branch. The benchmark-only render_when_occluded option was also removed rather than becoming framework API.

Platform boundary

Visibility is best-effort and inherits winit platform support. Product simulation remains deterministic because fixed ticks continue even when presentation is hidden.

Validation

• cargo test --manifest-path engine/Cargo.toml -p pocket-widget — 14 passed
• cargo clippy --manifest-path engine/Cargo.toml -p pocket-widget --all-targets -- -D warnings
• cargo check --manifest-path engine/Cargo.toml --workspace --all-targets
• live macOS Pocket Persona bridge smoke: Hide → hidden health → Show → render recovery
• git diff --check