[/find plugin] Hologram previews above shop signs (per-chunk, per-shop + per-player toggle)

Part of PAR-166 (spin out /find). Plugin/presentation — belongs in the new spun-out repo; filed under ChestShop until that project exists.

Port the legacy chestshop-database hologram feature: a floating item-display above each shop sign, loaded/unloaded per chunk, togglable per shop (owner) and per player (viewer). Legacy impl: PreviewHandler, PreviewListener, rendered on ShopCreated and chunk load, destroyed on ShopDestroyed.

Reads the hologram flag + player preview preference from the Treasury data layer (PAR-168) and shop locations via the spatial read API (PAR-170).

Includes the player-facing toggle commands (/find toggle preview <true|false>, /find toggle visibility <true|false>) writing through MarketApi (PAR-167 visibility, PAR-168 preview pref).

Depends on: PAR-168, PAR-170, PAR-167.