Add CivScheduler facade and migrate civmodcore sites

[grepsedawk]

Foundation for Folia readiness — touches civmodcore, the linchpin imported by ~31 plugins. Already reviewed once on the fork (grepsedawk#3); review hardening is folded in.

What

civmodcore had no scheduling abstraction: 13 call sites used Bukkit.getScheduler() / BukkitRunnable directly, which throw under Folia's regionized threading.

• New vg.civcraft.mc.civmodcore.scheduling.CivScheduler — runGlobal / runRegion(Location|Block) / runEntity / runAsync, each with Later and Timer variants returning a cancellable CivTask.
• Delegates directly to the region-aware schedulers Paper exposes on every build (getGlobalRegionScheduler / getRegionScheduler / getAsyncScheduler / Entity#getScheduler). On regular Paper these run on the main thread; on Folia they regionize. No reflection, no runtime server-type detection — one shadow jar runs on both. (API presence verified against paper-api 1.21.8 via javap.)
• Migrated all 13 civmodcore call sites. Block/entity work routes to the region/entity schedulers so it stays correct when regionized.

Thread-safety hardening

Moving these sites onto region schedulers means a global-thread timer now touches state that region/entity-thread event handlers also write. Made those structures concurrent:

• DoubleInteractFixer, BottomLine, CivScoreBoard: TreeMap → ConcurrentHashMap (UUID keys, no ordering relied upon).
• PlayerNames: HashSet → ConcurrentHashMap.newKeySet().
• TickCoolDownHandler: tickCounter → AtomicLong.

Plus two latent bugs the migration exposed: DelayedItemDrop mutated the caller's Location cumulatively in its drop loop (clone once); CivTask.wrap wrapped a null handle when the entity scheduler declined a retired entity (return a no-op task).

Tests

CivTaskTests (wrap/cancel/no-op). ./gradlew :plugins:civmodcore-paper:test BUILD SUCCESSFUL; existing suites pass. No Bukkit.getScheduler() / BukkitRunnable remain in main source outside the facade.

Notes for reviewers

• Adds a small public API on a heavily-imported module; the facade overload set is deliberately complete to serve the upcoming downstream migrations.
• aikar taskchain (used only by jukealert) is out of scope — not Folia-safe, but the facade only replaces direct Bukkit-scheduler sites.
• Two pre-existing Folia concerns preserved as faithful mappings, flagged for follow-up: GlobalChunkMetaManager iterates all worlds/loaded chunks from the global region (cross-region iteration isn't thread-safe on Folia); and the same race class extends to a few collections adjacent to the migrated ones (BottomLineAPI.lines, ScoreBoardAPI registries, TickCoolDownHandler.cds, the inner list in DoubleInteractFixer) — worth a dedicated pass.

This is the gate for migrating the ~300 scheduler sites across dependent plugins.