Implement config-driven unlock groups

Lift unlocking into first-class unlock groups defined in unlocks.toml, so
one defence-station drop can grant a bundle of ships/modules/buildings/
recipes at once (e.g. the salvager module + salvage bay), and buildings
can now be locked at game start and unlocked via a drop.

Config:
- New UnlocksConfig + ConfigLoader::loadUnlocks + validateUnlocks (unknown/
  duplicate/non-assembler grants, unknown requires, empty group all fail load).
- Drop unlock_at_station_level/unlock_requires from ship/module/recipe defs;
  add unlocked_at_start bool to recipes for graph-unreachable base recipes.

Simulation:
- Track awarded groups + per-building unlock state; an item starts unlocked
  iff no group grants it. Rework generateSchematicChoices/applySchematicChoice
  to operate on groups (grant all members at once). Extend state checksum.
- isBuildingUnlocked query; tryPlaceBuilding rejects locked types.

UI:
- Build-menu buttons for locked buildings start hidden, revealed via a new
  UnlockedBuildingsChangedEvent (emitted from GameWorldView's poll loop).
- Schematic choice dialog lists a group's granted items.
- Blueprint placement excludes locked building types (REQ-LOCK-UI-BLUEPRINT).

Data: unlocks.toml (app + test) reproduces the prior per-item gates as
singleton groups and adds salvage_operations (salvager + salvage_bay, lvl 1)
and reprocessing (reprocessing_plant, lvl 2).

Tests: rewrote unlock/config/recipe-schematic/artifact/wave/shipyard tests
for the group model; added UnlockGroupTest. All 443 cases pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y7N59FsLA5e2kuVdqe4Uhc

src/test/ShipyardTest.cpp

@@ -1,5 +1,7 @@
 #include "catch.hpp"
 
+#include <algorithm>
+
 #include "Building.h"
 #include "BuildingSystem.h"
 #include "BuildingType.h"
@@ -20,11 +22,24 @@ static GameConfig loadConfig()
     return ConfigLoader::loadFromDirectory(CONFIG_DIR);
 }
 
+// A ship starts unlocked iff no unlock group grants it (REQ-LOCK-EXPLICIT).
+static bool startsUnlocked(const GameConfig& cfg, const std::string& shipId)
+{
+    for (const UnlockGroupDef& group : cfg.unlocks.groups)
+    {
+        if (std::find(group.ships.begin(), group.ships.end(), shipId) != group.ships.end())
+        {
+            return false;
+        }
+    }
+    return true;
+}
+
 static const ShipDef* findAvailableSchematic(const GameConfig& cfg)
 {
     for (const ShipDef& def : cfg.ships.ships)
     {
-        if (def.unlockAtStationLevel == -1 && !def.schematic.materials.empty())
+        if (startsUnlocked(cfg, def.id) && !def.schematic.materials.empty())
         {
             return &def;
         }