fix: count active Crowd agents without null writes

[xsploit]

What

Fixes Crowd.getActiveAgentCount() by iterating Crowd slots and counting active agents directly.

Why

The current wrapper calls:

crowd->getActiveAgents(NULL, crowd->getAgentCount())

dtCrowd::getActiveAgents writes each active agent pointer into the supplied output array. Passing NULL therefore writes through a null pointer whenever the Crowd contains an active agent. In WASM this can corrupt low linear memory instead of failing at the call site.

Testing

• yarn build:packages
• yarn test (6/6 tests plus CJS/ESM node smoke tests)
• regression fills a 512-agent Crowd, verifies the active count, removes every agent, and verifies zero

This is independent of polygon-reference width and TileCache streaming.