Harden frame buffering and point-cloud memory use

[Lyptik]

Audit fixes

This implements the remaining findings from the ThreeJS example audit.

Tracking-frame handoff

• keep one pending frame per Scene
• continuous tracking state remains latest-wins
• preserve/conflate cluster transition states across replaced frames
• accumulate zone enter/leave counters with byte-range saturation
• never retain an old point cloud just to preserve a transient event

Point-cloud memory

• stop attaching SDK point arrays directly to Three.js BufferAttributes
• own renderer-side Float32Array buffers so a point cloud cannot retain the complete decompressed WebSocket frame backing store
• reuse capacity in 256-point chunks across small point-count changes
• use drawRange for the valid point count
• when capacity must grow, replace the geometry and dispose the old geometry so GPU resources are released

Validation/docs

• add frame-conflation regression tests
• add a generic UI-map/DOM selector contract test
• update README for Zstd compression, primary Velocity vectors control, bounded frame buffering and reusable point buffers
• record the memory/backpressure invariants in AGENTS.md

The stale Display Advanced preference reference was already fixed on main in commit 91224a2, so this PR is based after that fix.