Add platformer statechart example

[SandroMaglione]

What changed

• Add a compact playable SVG platformer example driven by @typeonce/effect-machine.
• Model top-level locomotion, facing, and wall-contact regions in parallel.
• Model Airborne as nested parallel motion and airJump regions.
• Represent double-jump readiness entirely as GroundLock, WallLock, Ready, and Spent states with scoped timers.
• Coordinate wall and double jumps through typed internal events while keeping physics and rendering in a small adapter.
• Add a live statechart, state-local telemetry, controls, and standalone documentation/tooling.

Why this is stacked

This PR targets codex/fix-nested-parallel-targets because entering Airborne exercises the corrected complete nested-parallel target API. Once PR #7 merges, this PR can be retargeted to main without changing its example-only diff.

Verification

• pnpm --dir examples/platformer check
• TypeScript and Vite production build passed.
• Browser-tested ground jump, readiness timer, one-shot double jump, rejected third jump, diving with spent capability, and reset.
• No browser warnings or runtime errors.