Permission group templates for island members (LuckPerms-style)

[tastybento]

Spun off from #2908 (item 6).

Summary

Allow island owners to define named permission templates (groups of flag/rank settings) and assign players to a template, instead of hand-tuning every flag for every member/trusted player individually.

Motivation

Today the only granularity is the rank ladder (visitor → coop → trusted → member → sub-owner → owner) plus per-flag rank thresholds set on the island. Owners who want, say, "builders" with break/place but no PvP, plus "guards" with PvP but no break, must repeatedly re-tune flags whenever a new player joins the equivalent group.

Original report:

Players can create their own permission group templates, just like the permission groups in LuckPerms. After setting up the template, the owner only needs to assign the template to trusted personnel to complete the configuration simply.
— Noogear, #2908

Sketch

• Owner defines a template: a name plus a Map<Flag, Integer> of effective rank values that override the island defaults for members of this group.
• Players are assigned to a template alongside (or instead of) a fixed rank.
• Flag checks resolve template-overrides before island-defaults.

Open questions

• Storage: per-island templates (each owner builds their own) or world-level templates owners pick from?
• Interaction with the existing rank ladder — does a template replace rank or layer on top of it?
• Permission/UI: does this need a new GUI panel, or extend the existing settings panel?
• Migration: existing islands keep working with no template assignments.

Scope

Likely a multi-PR effort: data model + persistence first, then API for addons, then UI. Not a small change.