Add Waystones compatibility (teleport + distance on sub-levels)

[ComfyFluffy]

Summary

Makes Waystones work with sub-levels. Without this, a waystone placed on an assembled sub-level (e.g. an aircraft) is broken in three ways: warping to it freezes the server, warping to an unloaded contraption is refused outright, and its distance/XP cost read as a meaningless ~20,000 km. After this PR you can place a waystone on a contraption and warp to it normally — loaded or unloaded.

All Waystones-specific code is a self-contained compatibility/waystones mixin package, auto-gated by SableMixinPlugin (no hard dependency — Sable loads fine without Waystones). Tested against Waystones 21.1.34 on NeoForge 1.21.1.

The problem

A waystone stores its block position, which for a block on a sub-level is the hidden plot-yard coordinate (DEFAULT_ORIGIN → ~20.48M blocks out). Waystones treats that as a normal world position:

• Freeze: the same-dimension warp path calls ServerGamePacketListenerImpl#teleport directly, bypassing Sable's existing ServerPlayer#teleportTo projection, so the player is flung into the plot-yard.
• Refusal: WaystoneImpl#isValidInLevel reads getBlockState(pos), which is air while the contraption is unloaded → "currently being moved or has gone missing".
• Bad distance: while unloaded there's no live pose to project against, so the plot-yard coordinate leaks into distance/cost on both server and client.

Changes

Core (used by the compat mixins, but generally applicable):

• SubLevelContainer now remembers a last-known pose and UUID per plot, set on allocate and kept while a sub-level is merely unloaded (cleared only on REMOVED). Persisted alongside plot occupancy in SubLevelOccupancySavedData. This lets a held sub-level's position/identity be resolved without loading it.
• ActiveSableCompanion#projectOutOfSubLevel falls back to that last-known pose when no sub-level is loaded at the position (previously it returned the raw plot-yard coordinate). This flows through distanceSquaredWithSubLevels → the Entity#distanceToSqr overwrite, so server-side distance/cost (is_within_distance, the distance cost variable, /waystones list) become correct for unloaded sub-levels.

Compat mixins (compatibility/waystones):

• WaystoneTeleportManagerMixin — projects the teleport target out of the sub-level; and if the target is on an unloaded contraption, freezes the player to (subLevelId, localAnchor) after the teleport.
• WaystoneImplMixin — treats a waystone on a reserved-but-unloaded plot as a valid teleport target.
• WaystoneButtonMixin (client) — omits the distance for a waystone whose sub-level isn't loaded here, reusing the same "no distance shown" path Waystones already uses for cross-dimension waystones.

How the unloaded warp works

It reuses the exact mechanism Sable already uses for bed respawns onto sub-levels: held sub-levels are stored in holding chunks keyed by their global world position, so teleporting the player to the contraption's last-known position re-activates it; the freeze then holds the player until the sub-level is live and places them precisely on the deck.

Testing

Manually, on a real modpack (Create: Aeronautics-style + Waystones, NeoForge 1.21.1):

• Warp to a waystone on a loaded aircraft → lands on the deck (previously froze the server).
• Warp to a waystone on an unloaded aircraft → world loads at its last position, player lands on the deck (previously "missing").
• XP cost → correct for both loaded and unloaded.
• GUI distance → real distance when loaded, blank when unloaded, unchanged for ordinary far-away waystones.

Notes for reviewers

• The projectOutOfSubLevel change looks wide but is provably safe for loaded sub-levels: subLevels[index] is written only in allocateSubLevel (set) and removeSubLevel (null), so getContaining never returns null during a loaded sub-level's lifetime — the new fallback branch is unreachable while loaded, and behaves identically across the load/unload boundary (lastKnownPose == logicalPose at unload). If you'd prefer it explicit, I'm happy to split it into a dedicated projectOutOfSubLevelOrLastKnown so the shared primitive is untouched.
• lastKnownUuids is new (respawn uses tracking points for identity instead). It's needed because waystones aren't proactively registered like beds, so we need a position→identity lookup for an unloaded sub-level.

Known limitations

• The warp trusts that a reserved plot still holds the waystone (no disk-read validation of the block while unloaded).
• Re-activation must land within the freeze window (~160 ticks); a very slow chunk load would drop the player at the contraption's last position.
• Client GUI distance is omitted (not computed) while the sub-level is unloaded; an accurate value would require streaming last-known poses to clients.

[CLAassistant]

All committers have signed the CLA.