Platform support: Folia and Purpur lanes (ArcLight/hybrids as stretch)

[PimvanderLoos]

Goal

Extend LightKeeper beyond Paper + Spigot with first-class test lanes for:

1. Folia — required to e2e-test plugins adding Folia support (AnimatedArchitecture wants this natively, soon).
2. Purpur — Paper derivative; expected to be near-free once the provisioner accepts alternative jar sources.
3. (Stretch, lower tier) ArcLight and similar hybrid/amalgamation servers.

Known work areas

• Provisioner/maven-plugin: jar acquisition + caching per platform (Folia/Purpur download APIs), platform-specific default configs, and lane wiring in lightkeeper-integration-tests (today: integration-paper + integration-spigot).
• Agent: AgentPlatformDetector needs Folia/Purpur signals; AgentMainThreadExecutor is built on BukkitScheduler#callSyncMethod, which does not exist on Folia — needs a region-scheduler-aware executor (this is the big one; "main thread" is not a single thread on Folia).
• Bot login driver: verify the reflective surface against Folia's jar (it tracks Paper mappings, so the SPLIT/SHARED class-name table should mostly hold — must be re-verified per the S6 surface-check methodology, .claude-documented in the AA repo).
• Event capture: Folia fires events on region threads; the capture listener and tick-stamp counter assumptions need auditing.
• Keep the fail-loud discipline: unsupported platform should be a clear provisioning error until a lane is genuinely supported.

Notes

• Purpur first (cheap), Folia second (the real work), hybrids only if demand materializes.

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