Large map: explored border as a continuous line along the fog edge

The game drew 2x2 px red dots per visited-sector neighbour (0x6c8580), in
64-tile steps and unrelated to the explored mask. Replaced by lines traced
from the explored cells (2x2 tiles), simplified to 0.75 px, rebuilt while
the map is open when the masks change.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -76,6 +76,7 @@ add_library(sacredK SHARED
     src/mods/screen.cpp
     src/mods/characters.cpp
     src/mods/mainmenu.cpp
+    src/mods/mapborder.cpp
     src/mods/mapreveal.cpp
     src/mods/questbook.cpp
     src/mods/followers.cpp