📝 update changelog for mine building (Issue #42)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CHANGELOG.md

@@ -7,6 +7,9 @@ Format follows [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 ## [Unreleased]
 
+### Added
+- **Mine Building** (Issue #42): 3×2 building placeable only on resource-free ROCK tiles (costs: 200 wood + 50 stone); Nisse with mine priority walk to the entrance, disappear inside for 15 s, then reappear carrying 2 stone; up to 3 Nisse work simultaneously; ⛏ X/3 status label shown directly on the building in world space; surface rock harvesting remains functional alongside the building
+
 ### Fixed
 - **Debug-Panel überlagert Nisse-Info-Panel** (Issue #41): F3-Debug-Panel weicht dynamisch aus — wenn das Nisse-Info-Panel offen ist, erscheint das Debug-Panel unterhalb davon statt darüber
 - **Stockpile-Overlay Transparenz** (Issue #39): `updateStaticPanelOpacity()` verwendete `setAlpha()` statt `setFillStyle()` — dadurch wurde die Opacity quadratisch statt linear angewendet; bei 100 % blieb das Panel sichtbar transparent