T3 audit gap 1 (BLOCKER). The server's ModifierKindIdSchema was still
z.enum([built-ins]) even after the client-side mirror was widened
during T29. Effect: any multiplayer room.create or
modifier-profile.update carrying a profile whose perType[i].kind is a
user-authored CustomModifierId (e.g. 'custom:shield') was rejected
server-side as INVALID_MESSAGE.
Symmetric fix: accept any non-empty string, defer kind validity to the
engine's registry-dispatch fallback at apply time (MODIFIER_REGISTRY
→ engine.customModifiers → warn-and-skip). The built-in id list is
preserved as a doc comment + void-referenced constant so it can't
silently drift.
Test updated: 'rejects a profile with unknown modifier kind' inverted
to 'accepts a profile with arbitrary modifier kind (T3 widening)' +
new 'rejects empty-string modifier kind' to preserve the min(1) guard.
Joey Yakimowich-Payne
7cc9617f3e