perf(scheduler): cache immutable dispatch plans

[Arbousier1]

Summary

• cache immutable global, region, and entity scheduler dispatch plans by runtime class
• use fixed-arity Sparrow invokers and flatten Paper fallbacks without weakening identity-safe concurrent routing
• cache Bukkit scheduler fallbacks by server identity and add mixed-capability, invalidation, concurrency, and no-double-scheduling coverage

Test plan

• ./gradlew test --tests 'top.ellan.mahjong.runtime.ServerScheduler*' -PmahjongJavaToolchain=25 -PmahjongJavaTarget=21
• ./gradlew spotlessCheck check assemble -PmahjongJavaToolchain=25 -PmahjongJavaTarget=21 -PmahjongRequireNative=true
• Local scheduler JMH: Folia 14.132 -> 4.182 ns/op (-70.4%), Paper 5.199 -> 4.099 ns/op (-21.2%), allocation stable at ~16 B/op

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