Playground: make travel and rotation readable with a floor reference

[a-baran-orhan]

Focused split of Workstream 5 from the dance UAT umbrella #91.

Goal

Give the playground a subtle floor reference so viewers can judge distance, path, and facing during travel and turn.

Tasks

• Add an optional, subtle floor reference that exposes orientation and distance without overwhelming the figure.
• Evaluate a grid, concentric stage markings, sparse metre markers, an origin marker, and a facing arrow.
• Consider a toggleable root path or foot-contact trail for travel/turn debugging.
• Show enough scale information to interpret the metre-based travel command.
• Test readability from front, side, three-quarter, and mobile camera views.
• Keep the default presentation suitable for non-technical viewers; provide a stronger debug mode for authors.

Acceptance

A viewer can tell the initial facing, turn direction, approximate distance traveled, and return-to-origin behavior without reading the source.

Priority: P1.

Note: a floor-guide module already exists (posecode-render/floor-guide.ts, buildFloorGuideData/createFloorGuide) and the playground has floor-guide keys — assess and extend the current implementation rather than starting from scratch.