Summary

Implements #1332.

• /bbox placeholders (alias ph) — player-only command that opens a hierarchical GUI showing all registered placeholders grouped by source (one button per addon). Clicking an addon opens a drill-down tree navigator that splits keys on _ and applies path compression to avoid empty intermediate folders. Layout is customisable via BentoBox/panels/placeholder_panel.yml and placeholder_list_panel.yml.
• /bbox dump-placeholders — console/admin command that writes plugins/BentoBox/placeholder-dump.md, a Markdown table of every placeholder and its plain-English description, suitable for the BentoBoxWorld/docs site. Numeric series are collapsed (e.g. 50 island_member_name_N rows → one row with range annotation).

Data-layer changes

• BasicPlaceholderExpansion stores a description alongside each replacer and tracks a disabled-placeholder set for in-memory toggle (state resets on restart).
• PlaceholderHook / PlaceholderAPIHook gain description-aware overloads, enable/disable API, and introspection methods (getRegisteredPlaceholders, getAddonsWithPlaceholders, getDescription). Also fixes a bug where only the first placeholder per addon expansion was tracked.
• PlaceholdersManager exposes the same API publicly.
• GameModePlaceholder constants now carry a plain-English description string shown in the GUI and dump.

Scalability (handles addons with thousands of placeholders)

• PlaceholderGrouper collapses numeric-suffix series (island_member_name_1..50 → one Series item).
• PlaceholderNode builds a _-split trie with path compression (single-child FOLDER chains are merged) for efficient tree navigation.
• PlaceholderListPanel uses the trie for drill-down with five node types — LEAF / FOLDER / LEAF_FOLDER / SERIES / LEAF_SERIES — each with distinct icons (PAPER / CHEST / BOOK / NAME_TAG / WRITABLE_BOOK) and left/right-click semantics.

GUI details

• Panel title shows the current node label only (not the full breadcrumb path) to stay within Minecraft's title width limit; colour is &3 (dark aqua) for readability against the grey inventory background.
• Long placeholder descriptions are word-wrapped at 38 characters per lore line.
• Each placeholder item shows its current resolved value for the viewing player (→ <value> or → (empty)). Series items show up to 3 sample values (#1 → …, #2 → …, #3 → …).
• The BentoBox core source button is hidden when no core placeholders are registered (currently always the case — all placeholders live under addon expansions).
• Clicking a placeholder or series toggles it enabled/disabled in-memory (disabled placeholders return "" instead of their resolved value).

Test plan

• All existing tests pass (./gradlew test)
• 18 new tests for PlaceholderGrouper (grouping, series detection, description stripping, sorting)
• 19 new tests for PlaceholderNode (trie construction, all 5 node types, path compression, counting, drill-in navigation)
• /bbox placeholders opens the browser GUI in-game; addon buttons show correct icons and counts
• Clicking an addon opens the per-source tree; clicking a FOLDER drills in; BACK navigates up; breadcrumb in title updates
• LEAF: click toggles (glows when disabled); value shown in lore
• SERIES: click toggles all members; up to 3 sample values shown
• LEAF_FOLDER: left-click toggles leaf, right-click drills in
• LEAF_SERIES: left-click toggles leaf, right-click toggles all series members
• /bbox dump-placeholders writes placeholder-dump.md with collapsed series rows and range annotations
• Panel YML files can be customised by editing files in BentoBox/panels/

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