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>

src/mods/questfail.cpp

@@ -0,0 +1,90 @@
+// Failed quests can be restarted (request of 2026-10-17): a failed quest is not
+// final, the quest giver offers it again.
+//
+// Measured (docs/quests.md): a failed quest keeps flags 3 (triggered + done) in
+// the quest table and its quest book entry gets state 101; Quest_Trigger
+// (0x46de10) does nothing for a quest with flag 1, so the giver never offers it
+// again. The game itself repeats only the generic quests (ids 2600..3599,
+// Quest_End copies them under a new id).
+//
+// The mod: after Quest_End failed a quest (its failure script has run), the
+// quest's flags are cleared and its book entries removed, as if it had never
+// been triggered. Quests that failed before the mod are reset the same way once
+// after loading.
+
+#include <algorithm>
+#include <cstdint>
+
+#include "game/engine/events.h"
+#include "game/engine/quests.h"
+#include "re/hook.h"
+#include "re/log.h"
+
+namespace mods::questfail {
+namespace {
+
+constexpr uint32_t kResetFlags = game::kQuestTriggered | game::kQuestDone | game::kQuestInProgress | game::kQuestBusy;
+
+bool g_resetAfterLoad = false;
+
+// The quest at table `index` as if it had never been triggered.
+void Restart(int index) {
+    uint8_t* quest = game::Quest(index);
+    const int32_t id = game::QuestId(quest);
+    re::Log("questfail: restarting quest %d '%s' (flags %x)", id, game::QuestName(quest), game::QuestFlags(quest));
+    game::QuestFlags(quest) &= ~kResetFlags;
+    for (size_t i = game::BookCount(); i-- > 0;) {
+        if (game::BookQuestId(game::BookEntry(i)) == id) {
+            game::EraseBookEntry(i);
+        }
+    }
+}
+
+void RestartFailedInBook() {
+    for (size_t i = game::BookCount(); i-- > 0;) {
+        const uint8_t* entry = game::BookEntry(i);
+        if (game::BookState(entry) != game::kBookFailed) {
+            continue;
+        }
+        const int index = game::QuestIndex(game::BookQuestId(entry));
+        if (index >= 0) {
+            Restart(index);
+        }
+        i = std::min(i, game::BookCount());
+    }
+}
+
+void OnTick(uint8_t*) {
+    if (g_resetAfterLoad) {
+        g_resetAfterLoad = false;
+        RestartFailedInBook();
+    }
+}
+
+void OnLoaded() {
+    g_resetAfterLoad = true;
+}
+
+}  // namespace
+
+// Quest_End (0x46ced0, quest manager thiscall (index, ?, success), ret 0xc):
+// LoseQuest passes success 0, ExitQuest 1.
+void __fastcall Quest_End(uint8_t* self, void* edx, int index, int a2, char success);
+RE_HOOK(0x0046ced0, Quest_End);
+void __fastcall Quest_End(uint8_t* self, void* edx, int index, int a2, char success) {
+    Quest_End_original(self, edx, index, a2, success);
+    if (!success && index >= 0 && static_cast<size_t>(index) < game::QuestCount()) {
+        Restart(index);
+    }
+}
+
+namespace {
+struct Register {
+    Register() {
+        game::OnEngineTick(&OnTick);
+        game::OnSaveGameLoaded(&OnLoaded);
+    }
+} g_register;
+}  // namespace
+
+}  // namespace mods::questfail