fix: prevent ~2.5-min world-exit freeze after teleporting far along the train

[bh679]

Summary

Leaving a Dungeon Train world (go-to-title / Board anew / Quit Game) after the player
has teleported/travelled far along the +X corridor froze the client for ~158 s before it
recovered, ending with DeathScreenLayout: pre-drain wait timed out →
TimeoutException at preDrainTrainSubLevels.

Root cause — a pre-existing Sable teardown hang (same class as the known Save-and-Quit hang
= Sable #679). The world-exit pre-drain busy-spun container.tick() for ~1 s/level but never
reflectively evicted the PlotChunkHolders that leak past that drain, so the integrated-server
stopServer wait loop spun on ChunkMap.hasWork() == true. The repo already had the proven fix,
but only on the ServerStopping path (ShipShutdownEvents). This makes the world-exit path use
the same hardened drain.

Changes

• New event/TrainSubLevelTeardown.java — the proven Sable feat: Books-written tally on the death screen #679 drain extracted from
  ShipShutdownEvents: delete trains → bounded pump → reflective PlotChunkHolder sweep →
  vanilla-holder drain (after save(flush=true)). Shared by both teardown paths.
• event/ShipShutdownEvents.java — refactored to a thin ServerStopping subscriber delegating
  to the helper (no behaviour change; ~249 → ~80 lines).
• client/DeathScreenLayoutHandler.java — preDrainTrainSubLevels now runs the hardened drain
  with a 3 s global server-thread pump budget (under the existing 10 s client give-up). The
  sweep/drain run even when pump is squeezed to 0 — that's what lets hasWork() go false. Covers
  all three callers (go-to-title, board-anew, quit-to-desktop).
• train/Trains.java — byTrainId log demoted DEBUG → TRACE (was ~471 lines/s during the
  teleport burst).

Mod-impact

No new deps, no version/mixin/registry/worldgen/networking changes. Touches Sable teardown timing
only. Reflective updatingChunkMap access is the same already-shipped technique as
ShipShutdownEvents/ShutdownDiagnostics (try/catch → warn, never throws).

Test plan

• ./gradlew build — green; all 88 unit-test classes pass.
• ./gradlew runClient — loads clean to title; a real world-exit ran the new path and
  drained 2360 vanilla ChunkHolders with no pre-drain wait timed out and an immediate
  ServerStopped.
• Train-loaded repro (/tp @s ~3000 ~ ~, ride to spawn carriages, then go-to-title) — expect
  swept N PlotChunkHolders and a ~1–3 s return to title (no freeze). Sable: exercised on the
  moving train.

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