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/lib/config/RecipesConfig.h

@@ -32,14 +32,12 @@ struct RecipeDef
     std::vector<RecipeIngredient> inputs;
     std::vector<RecipeOutput> outputs;
     double durationSeconds;
-    // Assembler only. nullopt = implicit-only locking. -1 = explicitly unlocked
-    // at game start. >= 0 = locked; schematic enters drop pool at that station
-    // level once the output item is implicitly unlocked (REQ-LOCK-EXPLICIT).
-    std::optional<int> unlockAtStationLevel;
-    // Assembler recipe schematics only. Prerequisite schematic ids that must be
-    // explicitly unlocked before this schematic can enter the drop pool
-    // (REQ-LOCK-PREREQ). Empty = none.
-    std::vector<std::string> unlockRequires;
+    // Assembler only. When true, this recipe is available from game start
+    // regardless of the implicit item graph — used for base recipes that no
+    // schematic's materials reach (e.g. building blocks). See REQ-LOCK-IMPLICIT.
+    // Otherwise an assembler recipe is either explicitly gated (granted by an
+    // unlock group, REQ-LOCK-EXPLICIT) or implicitly gated via the item graph.
+    bool unlockedAtStart = false;
 };
 
 struct RecipesConfig