feat(tree-feller): migrate distribution and local harness to Purpur

BREAKING CHANGE: New releases require Java 25/Purpur 26.2 and use purpur-tree-feller-<version>.jar. Runtime TreeFeller identity, data folder, progression and felling/undo behavior are unchanged. Replace the old JAR rather than installing both distributions. The local harness now selects Purpur 2618; its stored worlds and operator/port/RCON settings are preserved. No containers are started by this migration.

scripts/start-test-server.sh

@@ -32,7 +32,7 @@ if [[ -z "$container_id" ]]; then
   exit 1
 fi
 
-echo "Waiting up to ${wait_seconds}s for Spigot 26.2..."
+echo "Waiting up to ${wait_seconds}s for Purpur 26.2 build 2618..."
 deadline=$((SECONDS + wait_seconds))
 while (( SECONDS < deadline )); do
   container_state="$(docker inspect --format '{{.State.Status}}' "$container_id")"
@@ -51,7 +51,7 @@ done
 health_state="$(docker inspect --format '{{if .State.Health}}{{.State.Health.Status}}{{else}}{{.State.Status}}{{end}}' "$container_id")"
 if [[ "$health_state" != "healthy" ]]; then
   docker compose -f "$compose_file" logs --tail=150 minecraft >&2
-  echo "Timed out waiting for Spigot 26.2 (status: $health_state)." >&2
+  echo "Timed out waiting for Purpur 26.2 build 2618 (status: $health_state)." >&2
   exit 1
 fi