Summarize 1.20.1 vs 1.21.1 specs and tighten selector/cache behavior

[Copilot]

Pre-0.2.0 QA requires a concise summary of 1.20.1 vs 1.21.1 specification differences and a focused review of outstanding implementation risks. This change documents the version deltas and applies targeted fixes to selector ordering and UI/registry lookup hot paths.

• Spec differences handoff
  • Add a release-facing 1.20.1 vs 1.21.1 differences summary (loader/API, Replication, Mekanism, datapack paths).
• Selector ordering + persistence guards
  • Stabilize equal-priority rule ordering and fail fast on non-static selector persistence; add coverage tests.
• Client and resolver performance
  • Cache MatterIndexScreen lookups, restore NeoForge search tick, and add synchronized/volatile caches for mod-scoped node resolution.

Example:

val orderedRules =
    rules.withIndex().sortedWith(
        compareBy<IndexedValue<MatterRule>> { it.value.source.priority }
            .thenBy { it.index },
    )

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• maven.fabricmc.net
  • Triggering command: /usr/lib/jvm/temurin-17-jdk-amd64/bin/java /usr/lib/jvm/temurin-17-jdk-amd64/bin/java --add-opens=java.base/java.lang=ALL-UNNAMED --add-opens=java.base/java.lang.invoke=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.api=ALL-UNNAMED --add-exports=jdk.compiler/com.sun.tools.javac.util=ALL-UNNAMED --add-opens=java.base/java.util=ALL-UNNAMED --add-opens=java.prefs/java.util.prefs=ALL-UNNAMED --add-opens=java.base/java.nio.charset=ALL-UNNAMED --add-opens=java.base/java.net=ALL-UNNAMED --add-opens=java.base/java.util.concurrent=ALL-UNNAMED --add-opens=java.base/java.util.concurrent.atomic=ALL-UNNAMED --add-opens=java.xml/javax.xml.namespace=ALL-UNNAMED --add-opens=java.base/java.time=ALL-UNNAMED -Xmx1G -Dfile.encoding=UTF-8 -Duser.country -Duser.language=en -Duser.variant (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)