refactor(domain): extract chapter classification heuristics from form.py

Moved supplement_score, should_preselect_chapter, and
ensure_at_least_one_chapter_enabled to domain/chapter_classification.py.

Also moved coerce_bool from settings.py to common.py to break circular
import introduced in previous commit.

1025 tests pass.

tests/test_chapter_classification.py

@@ -0,0 +1,97 @@
+"""Tests for domain/chapter_classification.py."""
+
+from abogen.domain.chapter_classification import (
+    supplement_score,
+    should_preselect_chapter,
+    ensure_at_least_one_chapter_enabled,
+)
+
+
+class TestSupplementScore:
+    def test_title_page_high_score(self):
+        score = supplement_score("Title Page", "", 0)
+        assert score > 3.0
+
+    def test_chapter_title_negative_score(self):
+        score = supplement_score("Chapter 1", "", 0)
+        assert score < 0
+
+    def test_copyright_high_score(self):
+        score = supplement_score("Copyright", "All rights reserved.", 0)
+        assert score > 2.0
+
+    def test_short_text_adds_score(self):
+        score = supplement_score("Some Title", "Short text.", 0)
+        assert score > 0.5
+
+    def test_long_text_low_score_contribution(self):
+        score = supplement_score("Some Title", "word " * 200, 0)
+        assert score < 0.5
+
+    def test_index_zero_bonus(self):
+        score_title = supplement_score("Dedication", "For my family.", 0)
+        score_other = supplement_score("Dedication", "For my family.", 3)
+        assert score_title > score_other
+
+    def test_empty_title_and_text(self):
+        score = supplement_score("", "", 5)
+        assert score == 0.9  # short text bonus only (len("") ≤ 150)
+
+    def test_newsletter_keyword_high_score(self):
+        score = supplement_score("Subscribe", "Join our newsletter today", 0)
+        assert score > 3.0
+
+    def test_acknowledgments_pattern(self):
+        score = supplement_score("Acknowledgements", "", 0)
+        assert score > 2.0
+
+    def test_glossary_in_title(self):
+        score = supplement_score("Glossary", "", 0)
+        assert score > 2.0
+
+
+class TestShouldPreselectChapter:
+    def test_single_chapter_always_preselected(self):
+        assert should_preselect_chapter("Anything", "", 0, 1) is True
+
+    def test_chapter_preselected_when_low_score(self):
+        assert should_preselect_chapter("Chapter 1", "The story begins.", 0, 10) is True
+
+    def test_title_page_not_preselected(self):
+        assert should_preselect_chapter("Title Page", "", 0, 10) is False
+
+    def test_copyright_not_preselected(self):
+        assert should_preselect_chapter("Copyright", "All rights reserved.", 0, 10) is False
+
+    def test_toc_not_preselected(self):
+        assert should_preselect_chapter("Table of Contents", "", 0, 10) is False
+
+
+class TestEnsureAtLeastOneChapterEnabled:
+    def test_empty_list(self):
+        chapters = []
+        ensure_at_least_one_chapter_enabled(chapters)
+        assert chapters == []
+
+    def test_already_has_enabled(self):
+        chapters = [
+            {"title": "Ch1", "enabled": False},
+            {"title": "Ch2", "enabled": True},
+        ]
+        ensure_at_least_one_chapter_enabled(chapters)
+        assert chapters[1]["enabled"] is True
+        assert chapters[0]["enabled"] is False
+
+    def test_none_enabled_picks_longest(self):
+        chapters = [
+            {"title": "Ch1", "enabled": False, "characters": 100},
+            {"title": "Ch2", "enabled": False, "characters": 500},
+            {"title": "Ch3", "enabled": False, "characters": 200},
+        ]
+        ensure_at_least_one_chapter_enabled(chapters)
+        assert chapters[1]["enabled"] is True
+
+    def test_single_chapter_gets_enabled(self):
+        chapters = [{"title": "Only", "enabled": False}]
+        ensure_at_least_one_chapter_enabled(chapters)
+        assert chapters[0]["enabled"] is True