Add freshness bar, enhance overlays and renderers

- Add enemy freshness tracking (novelty bonus for repeated deploys)
- Add freshness bar to sidepanel enemy cards with penalty indicator
- Major overhaul of renderer-overlays.js (790+ lines for UI polish)
- Enhanced combat log, shop overlays, and inventory UI
- Improved weapon/upgrade display with partial ownership colors
- Added element icons and weakness/resistance indicators to cards
- Enhanced radial menu and tooltip system
- Add "stale/%" penalty text when freshness depleted
- Update play link to ffazeshift.net in index.html

js/upgrades.js

@@ -38,6 +38,8 @@ function buyTowerUpgrade(upgradeId) {
     addLog('Tower already at full HP.', 'info');
     return;
   }
+  // Tier requirement
+  if ((upgrade.minTier ?? 0) > (G.difficultyTier || 0)) return;
   for (const req of upgrade.requires) {
     if (!G.towerUpgradesBought.includes(req)) return;
   }
@@ -161,6 +163,7 @@ function buyWeaponUpgrade(instanceId, upgradeId) {
   if (!upgrade) return;
   const bought = G.weaponUpgradesBought[instanceId] || [];
   if (bought.includes(upgradeId)) return;
+  if ((upgrade.minTier ?? 0) > (G.difficultyTier || 0)) return;
   for (const req of upgrade.requires) { if (!bought.includes(req)) return; }
   if (spendableCredits() < upgrade.cost) return;
   G.credits -= upgrade.cost;