Add verified Folia 26.1.2 compatibility

[Minecraft0122]

Summary

• target the exact dev.folia:folia-api:26.1.2.build.8-stable API with Java 25 and declare api-version: '26.1' for BetterRTP and BetterRTPAddons
• replace legacy PaperLib chunk/teleport flows with Folia-aware region, entity, async, and teleport scheduling through FoliaLib
• keep forced-player commands, player messages, inventory cleanup, particles, delayed tasks, reloads, and shared player state on the correct scheduler or in thread-safe storage
• build both jars in CI and boot the shaded BetterRTP jar on the exact Folia 26.1.2 build 8 server, with PaperMC/Mojang artifacts pinned by SHA-256

This is intentionally a modern Folia-targeted build. Folia 26.1.2 requires Java 25.

Verification

• mvn -B -DskipTests clean install
• mvn -B -DskipTests clean package in BetterRTPAddons
• fork GitHub Actions run passed the full build, exact Folia runtime smoke test, Addons build, and artifact upload workflow
• clean startup, plugin enable, /brtp version, /brtp info, queue generation, and shutdown on Folia 26.1.2-8
• real player-protocol smoke test: delayed overworld RTP, live config reload, then an immediate cross-dimension RTP to the Nether; both teleports were confirmed by the client
• two-region concurrency smoke test with threaded-regions.threads=2: two players in regions about 5,000 blocks apart completed concurrent RTPs while background queue generation was active

No BetterRTP exceptions or Folia thread-ownership violations were observed in these runs.