Character panel: minus buttons take back attribute and skill points

- respec: the panel's unused second button array becomes − buttons between
  name and value; attributes give back invested points, skills give back
  skill points, a level-1 skill is removed and can be chosen anew; all
  through the game's own point functions (protected copy stays in sync)
- the − is the + icon with its vertical arm covered (no minus image exists)
- hud: RenderCentred / HandleCentred for mods hooking centred windows;
  mouse event helpers shared in game/ui/events.h

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

CMakeLists.txt

@@ -53,6 +53,7 @@ add_library(sacred_re SHARED
     src/mods/escape.cpp
     src/mods/recordcache.cpp
     src/mods/megamap.cpp
+    src/mods/respec.cpp
 )
 target_include_directories(sacred_re PRIVATE src)
 target_compile_options(sacred_re PRIVATE ${SACRED_RE_CXX_FLAGS})