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/ui/BuildButtonGrid.cpp

@@ -82,6 +82,7 @@ BuildButtonGrid::BuildButtonGrid(Simulation* sim, const GameConfig* config, QWid
             std::make_shared<DemolishModeToggleRequestedEvent>());
     });
 
+    updateVisibility();
     registerForEvents();
 }
 
@@ -119,6 +120,17 @@ void BuildButtonGrid::updateAffordability()
     }
 }
 
+void BuildButtonGrid::updateVisibility()
+{
+    // A locked building type's button is hidden until its unlock group is awarded
+    // (REQ-LOCK-BUILDING). Buttons keep their index so hotkeys/affordability stay
+    // stable; they simply appear once the type is unlocked.
+    for (std::size_t i = 0; i < m_buttons.size(); ++i)
+    {
+        m_buttons[i]->setVisible(m_sim->isBuildingUnlocked(m_types[i]));
+    }
+}
+
 void BuildButtonGrid::clearActiveButton()
 {
     if (m_activeIndex)
@@ -165,6 +177,12 @@ void BuildButtonGrid::handleEvent(std::shared_ptr<const BuildingBlocksChangedEve
     updateAffordability();
 }
 
+void BuildButtonGrid::handleEvent(std::shared_ptr<const UnlockedBuildingsChangedEvent> /*event*/)
+{
+    updateVisibility();
+    updateAffordability();
+}
+
 void BuildButtonGrid::handleEvent(std::shared_ptr<const DemolishModeChangedEvent> event)
 {
     m_demolishButton->setChecked(event->active);
@@ -176,9 +194,10 @@ void BuildButtonGrid::handleEvent(std::shared_ptr<const BuildHotkeyPressedEvent>
     {
         if (m_types[i] == event->type)
         {
-            // Equivalent to clicking the build button: a disabled (unaffordable)
-            // button cannot be clicked, so the hotkey is likewise inert.
-            if (m_buttons[i]->isEnabled())
+            // Equivalent to clicking the build button: a disabled (unaffordable) or
+            // hidden (locked, REQ-LOCK-BUILDING) button cannot be clicked, so the
+            // hotkey is likewise inert.
+            if (m_buttons[i]->isEnabled() && m_buttons[i]->isVisible())
             {
                 onBuildButton(static_cast<int>(i));
             }