freeze verb: pin the client-side things that make two shots differ

[Flatts3000]

The problem

Two screenshots of the same scene should differ only by the thing under test. Today they can differ
by weather, time of day, particle state, entity positions, and animation phase. Some of those are
commands (time set, weather clear), and some are not.

This is what stops a screenshot being an assertion rather than an illustration.

The proposal

A verb that pins the things commands cannot reach:

{"verb": "freeze", "enabled": true}
{"ok": true, "frozen": ["particles", "entityAnimation", "clouds", "weatherRendering"]}

Following the established rule: a bare toggle verb restores what vanilla does, false gives the
devbridge behaviour.

Why the mod and not a command

time set and weather clear are commands and stay out of scope. Particle spawning, cloud
scrolling and animation ticks are client render state with no command behind them.

Notes

• Overlaps with the settle verb: settle answers "has it finished drawing", freeze answers "will it
  keep changing". Both are needed for a reproducible shot and neither replaces the other.
• Worth deciding whether this pins entity AI too, which is server-side and arguably command
  territory via /tick freeze.