feat(practice): always-on spline/particle guide trail for Practice Mode

Description

SplineVisualizationSystem is promoted from a debug feature to a player-facing guide, automatically enabled during Practice Mode sessions. Lowest priority — safe to defer post-MVP.

Acceptance Criteria

• `SplineVisualizationSystem` activates automatically when `GameMode.PRACTICE` is detected
• Particle trail is visible to the practicing player only (not broadcast to other instances)
• Visual parameters are tunable per map: particle density, color, lookahead distance threshold
• Defaults are set for first-time-player readability (higher density than debug mode)
• The system is disabled in `GameMode.RACE` sessions (no performance cost in competitive play)
• Guide points must be authored in the setup plugin before the system has data to render
• Unit/integration tests verify the system activates and deactivates with the correct game mode
• No visual artifacts when a player retries (trail resets with the ring tracker)

Technical Details

• Guide point authoring in the setup plugin is a prerequisite (tracked separately)
• `SplineVisualizationSystem` remains in `server` module — no shared-module imports of Minestom
• Visual parameters stored in MapDefinition or a sidecar config file (Gson-serialized)
• System follows the ECS pattern: declares required components, processes matching entities each tick

Dependencies

• #172 (Practice Mode session — system only activates under `GameMode.PRACTICE`)

Estimate

L

Start with dependency #172 and inspect SplineVisualizationSystem in the server module, then review MapDefinition or the sidecar Gson configuration and the setup plugin prerequisite. Trace the ECS processing and ring-tracker reset behavior. Done means Practice Mode enables the player-only trail with tunable defaults, Race Mode disables it, and activation, deactivation, and retry behavior are covered by tests.