Halve lesson gain when the bag has no textbook for the subject.

Locker and home do not count. A pack without the field keeps vanilla 0.5.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/HSchool.Server/mods/core/defs/behavior/rules.jsonc

@@ -4,6 +4,8 @@
   "needThreshold": 0.35,
   // Skill points a lesson adds per game hour, before traits and need state.
   "lessonSkillPerHour": 0.05,
+  // No textbook for this lesson in the bag (locker and home do not count).
+  "lessonNoTextbookFactor": 0.5,
   // Inclusive extra commute minutes. 0–6 matches the previous hardcoded roll so
   // the same seed still arrives at the same minute.
   "commuteSlackMin": 0,

src/HSchool.Server/mods/core/localizations/en.jsonc

@@ -206,5 +206,6 @@
   "ApparelChanged": "changed clothes: {0}",
   "LessonNoTeacher": "lesson without a teacher: {0}",
   "LessonCold": "too cold in class: {0}",
+  "LessonNoTextbook": "no textbook: {0}",
   "core": "Core",
 }

src/HSchool.Server/mods/core/localizations/ru.jsonc

@@ -206,5 +206,6 @@
   "ApparelChanged": "переоделся: {0}",
   "LessonNoTeacher": "урок без учителя: {0}",
   "LessonCold": "замёрз на уроке: {0}",
+  "LessonNoTextbook": "нет учебника: {0}",
   "core": "Базовая игра",
 }