Failed quests restartable

A failed quest gets its flags cleared and its quest book entries removed
after its failure script ran, so the quest giver can offer it again; quests
failed before the mod are reset once after loading. Quest table and quest
book in game/engine/quests.h, measurements in docs/quests.md.

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

CMakeLists.txt

@@ -74,6 +74,7 @@ add_library(sacredK SHARED
     src/mods/screen.cpp
     src/mods/characters.cpp
     src/mods/mainmenu.cpp
+    src/mods/questfail.cpp
     src/mods/anticheat.cpp
 )
 target_include_directories(sacredK PRIVATE src)