fix(activities): route activities to queues by lookup, and check the fallback

[KillerX]

13/n of a stack. Base: refactor/execute-shared-body (#451). Closes Activity→queue routing is stringly-typed with a silent fallback.

Directly related to #451: GetQueueForActivity is the one call executeOnQueue makes to decide a queue, so that extraction concentrated everything on this function.

It walked three slices with lo.Contains on every Execute call to answer a question with a fixed answer. It is a map built once now.

Building it once makes the collision case detectable. Two structs defining the same method name would make the answer depend on the order of the three loops; it panics instead. Not a new class of failure — activities are registered under the same short name, the worker runs with DisableRegistrationAliasing, and the debug queue registers every struct, so the worker would refuse to start anyway. Failing in the builder says why.

The silent fallback cannot be removed. A name says nothing about what an activity needs, so an ffmpeg activity hung on the wrong struct lands on the worker queue and fails there as an opaque timeout — the audit's point. TestEveryFFmpegActivityIsOnAnFFmpegQueue reads the package source instead: it finds every exported activity method whose body reaches into the ffmpeg or transcode packages and asserts it hangs off Audio or Video. 38 methods qualify today, and the test requires a known one to be found so it cannot pass by looking at nothing.

One activity is already in that trap. CropShortActivity calls ffmpeg.GetStreamInfo from UtilActivities to choose between 25 and 50 fps, on a queue whose image has no ffmpeg — and it discards the error, so it does not fail, it quietly crops 50 fps material at 25. Allowlisted and written up in potential_improvements.md rather than moved here, because moving it changes which worker runs it. Two ways out, both wanting your call: move the probe into the video activity that runs the resulting arguments, or move the whole activity to VideoActivities.

Also added the test the audit asked for: no activity name defined on two structs. There are 112 and they are unique today.

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