Add block group limits

[tastybento]

Fixes #12 — the block half of group limits (entity groups shipped earlier). This is what #84 (piston + sticky piston) and #132 (grass→dirt conversion bypass) were closed into.

Config

blockgrouplimits:
  Pistons:
    icon: PISTON
    limit: 10
    materials:
    - PISTON
    - STICKY_PISTON
  Soil:
    icon: GRASS_BLOCK
    limit: 200
    materials:
    - GRASS_BLOCK
    - DIRT
    - DIRT_PATH
    - FARMLAND

blockgrouplimits-nether:
  Pistons: 5

Mirrors entitygrouplimits exactly: named group, icon, member list, shared limit, per-environment overrides (limit only). Member names normalise through the existing variant map, so e.g. a piston head resolves to its piston.

Behaviour

• At placement (and every path that funnels through process() — grow, spread, form), the counts of all group members are summed and checked against the group limit. Converting grass→dirt no longer escapes the limit because both count into the same pool.
• The individual-limit cascade (island → world → env default) runs first and still applies inside a group, so a tighter per-material limit wins.
• The recount tracks group members even when they carry no individual limit; the config comment tells admins to recount after adding a group.
• The limits GUI gains a row per group: icon, member list, summed count/limit with the usual colouring.

Notes / deliberate v1 scope

• The hit-limit message names the placed material with the group's limit value. Naming the group in the message (like entity groups do) needs process()'s int-return contract reworked across all state-transition handlers — worth doing separately if wanted.
• Permission-based block group limits (<gm>.island.limit.<groupname>.<n>) and group offsets are not included; both need IslandBlockCount data-model additions and can follow if there's demand.

Tests

Settings parsing (definitions, env overrides, unknown material, undefined-group override), group-full cancels without double-count, under-limit allows and counts, tighter individual limit still applies. Full suite green (262).

In-game test plan

1. Define the Pistons group above, restart, place 6 pistons + 4 sticky pistons.
2. The 11th of either kind is blocked; /is limits shows a Pistons row at 10/10.
3. Break one piston → a sticky piston can now be placed (9→10 again).
4. Set blocklimits: PISTON: 3 as well → the 4th piston is blocked even with the group at 7/10.
5. /is limits recount → group count matches reality (including blocks placed before the group existed).
6. Nether: with the -nether override at 5, verify the pool is 5 there and 10 in the overworld.

🤖 Generated with Claude Code

https://claude.ai/code/session_015dbJyrv2uxHfTmiWkqGUJB

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
66.7% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud