spike: measure parked-car streaming cost (go/no-go for view-gating)

[nigel-hall-codes]

Gate for the #202 view-based streaming chain. Decide GO/NO-GO before any code.

Scope

• Read how game hitches when streaming in new chunk #104 ("game hitches when streaming in new chunk", CLOSED) was resolved; note what already mitigates the hitch (paced spawns, etc.).
• Run the three zero-code measurements from sdlc/idea: view-based streaming — only spawn entities visible to the Prometheus camera #202/analysis.md in a dense, hilly area:
  1. ParkedCars A/B toggle → per-frame cost (draw calls, batches, tris, frame time).
  2. Profiler CPU timeline at chunk stream-in → instantiation cost (the hypothesis).
  3. Per-chunk parked-car census (from _parked.json or the placed-count log).
• Confirm the Prometheus/follow camera drives Camera.main.

Done = a decision: GO (real instantiation hitch remains) or NO-GO (already mitigated / trivial). On NO-GO, stop the chain.
Also decide whether this makes the chunk-size lever (#114) redundant for the entity layer or complementary.

Parent: #202

[nigel-hall-codes]

Spike complete — GO decision made. All child issues (#209, #210, #211) shipped.