Version content gate core

- Add the ContentGate, CompatWalker, PackCompatReport and finding types under core.compat
- Resolve block keys through the live registry, the legacy rename table, dimension blockFallbacks and the entry backup on every platform
- Move the legacy rename table into LegacyBlockRenames so Bukkit and the mod loaders apply it identically
- Evaluate every registrant on load from IrisData.preprocessObject and store the verdict on IrisRegistrant
- Add IrisDimension.blockFallbacks as a plain object property
- Keep missing palette keys as MissingBlockState placeholders so a chance-1 edit rule can still rewrite them
- Guard the modded rotator, merger and face fixer against non-modded states
- Source the legacy potion effect aliases from the gate so the Bukkit-typed table stays unloaded on modded

.gitignore

@@ -71,3 +71,6 @@ AGENTS.md
 build-lt.sh
 BUILDS/
 .run/
+
+# Version-content compat lane worktree. Generated, never source.
+.compatwt/