Fix MC 26.2 / Fabric API 0.152.2 build: strip world+HUD overlays, use new keymapping API

MC 26.2 overhauled rendering significantly. The classes we used don't
exist anymore:

- MultiBufferSource was removed; world line drawing now goes through
  SubmitNodeCollector via LevelRenderEvents.COLLECT_SUBMITS — a much
  larger rewrite. Defer until next iteration.
- HudLayerRegistrationCallback / IdentifiedLayer / LayeredDraw — replaced
  by HudElementRegistry / HudElement / VanillaHudElements. Compass and
  watermark deferred until next iteration.
- fabric-key-binding-api-v1 module renamed to fabric-key-mapping-api-v1.
  Class is now KeyMappingHelper at net.fabricmc.fabric.api.client.keymapping.v1.
- KeyMapping ctor: last arg is KeyMapping.Category, not a String.
- BlockPos#getCenter removed — construct Vec3 manually.
- ResourceKey#location renamed to identifier (returns Identifier).
- ResourceLocation was renamed to Identifier (in 26.1).

Friends/markers/chat/tab-complete still work; marker_set from friends
shows up as a chat line "[IRC] marker from X @ x, y, z" instead of an
in-world beacon for this iteration.

.gitignore

@@ -5,4 +5,5 @@ run/
 *.iml
 .idea/
 *.class
+.primer_*.md