ci(plugins): run the plugin self-tests and build the shipped plugin jars

The three framework-free test mains under plugins/*/test were never run by
anything — not CI, not the plugin builds — and the velocity/paper/limbo jars
were only ever compiled by deploy/bootstrap.sh on a live host. CI gains a
'plugins' job (JDK 21 plus the Gradle 8.14 the plugin Dockerfiles pin) running
plugins/test.sh: the three mains (InviteCardTest's jars fetched from Maven
Central, pinned and digest-checked) and the three production builds.

The first real run surfaced and fixed two untested assumptions: InviteCardTest's
documented javac line omitted examination-api (adventure-api's Component
signatures reference Examinable, so javac needs it too), and limbo's '+' version
default cannot resolve — LOOHP's repository serves no maven-metadata — so the
script resolves the current release off the Limbo CI artifact name (the same
source bootstrap reads) and plugins/README.md stops advertising a bare
'gradle -p plugins/limbo build' that can never work.

Verified in gradle:8.14-jdk21 on the VM: mains OK (32/36/48 checks);
velocity/paper/limbo BUILD SUCCESSFUL.

plugins/README.md

@@ -198,9 +198,12 @@ preference):
 # Velocity — system Gradle is fine
 gradle -p plugins/velocity build
 
-# Paper and limbo — system Gradle too, but both compile on a Java-21 toolchain (see table)
+# Paper and limbo — system Gradle too, but both compile on a Java-21 toolchain (see table).
+# limbo also needs the LOOHP/Limbo API release it compiles against: the module's `+`
+# default cannot resolve (LOOHP's repository publishes no maven-metadata), so pass the
+# release that matches the Limbo.jar you bundle, exactly as deploy/bootstrap.sh does:
 gradle -p plugins/paper build
-gradle -p plugins/limbo build
+gradle -p plugins/limbo build -PlimboVersion=<release, e.g. 2026.0.3-ALPHA>
 
 # Fabric / Forge / NeoForge — use the per-module wrapper. Nothing installs these; the jar you
 # want is the one this produces.