Fix paragraph breaks, drop caps, headers and captions for chapters 2-6 of the Clark book

Root cause: clean-up found paragraph breaks from the layout indent of pdftotext -layout
(columns, relative to the smallest indent in the whole chapter). Chapter 1 happened to
have a clean left margin; later pages are skewed and drift 1-2 columns, drop caps push
the first lines right by varying amounts, and some pages wrap text around a label, so
continuation lines were taken as paragraph starts (a blank line inside a sentence).

Fix (all deterministic, all logged to warnings.log):
- pdftext: read each line's position in points (pdftotext -bbox-layout) and attach it to
  the layout line it belongs to (units() now carries 'geom'; merged/partial lines get none).
- cleanup: a paragraph starts when a line sits >= 4.5 pt right of the line before it, or
  >= 2.5 pt lower than the page's normal line pitch. A line without a position or starting
  in lower case never starts a paragraph. Text wrapped beside a drop cap is not a start.
  Without positions the old indent rule is still used.
- cleanup: drop cap + small caps on ONE layout line ('F     OR MANY YEARS') is repaired like
  the two-line form (chapters 4 and 5).
- pageparts.header_zone: the running header is found even when text follows it without a
  blank line, and the right-hand header (chapter title, page number sometimes misread as
  't5' or 'a') is recognised from the chapter title.
- cleanup: all-capitals lines below the text are captions (removed); an all-capitals line
  above the text is kept as a section heading in title case ('The Deferred Preface');
  'Right: MARK TWIN' style captions removed; '5. JOY' labels set inside a sentence removed.
- cleanup: paragraph starting with a lone quote mark + lower-case word joins the paragraph
  before it; paragraph-leading ' + capital becomes '"' (not elisions such as 'Tis); a bullet
  in place of an opening quote becomes '"'; closing quote with no opening quote, and a '1'
  that is probably the letter I, are logged only.
- cleanup: mid-line 'worth- while' joined only when certain, otherwise logged.
- tests: new _pipeline/tests/test_cleanup.py (36 tests, 27 fail on the previous code);
  test_pipeline.py helper test_piece renamed make_test_piece so pytest stops collecting it.

Chapter 1 text is byte-identical to the approved pilot text.
Re-run for chapters 1-6: cleanup, figures, script, license. Text, script, warnings and
book.json for chapters 2-6 added.

_pipeline/pageparts.py

@@ -16,20 +16,28 @@ def similar(a, b):
     return SequenceMatcher(None, norm(a), norm(b)).ratio()
 
 
-def header_zone(lines, title):
+def _mostly_upper(text):
+    letters = [c for c in text if c.isalpha()]
+    return (not letters) or sum(c.isupper() for c in letters) / len(letters) > 0.7
+
+
+def header_zone(lines, title, extra_titles=()):
     """The running header at the top of a unit.
     Returns (first_line, last_line_exclusive, page_number or None) or None.
-    A header is the first block of lines (up to a blank line) that either carries a page number
-    next to uppercase text, or reads as the book title."""
+    The header starts at the first non-empty line and runs over the following lines while they are
+    upper case (at most 3 lines in all), whether or not a blank line separates it from the text.
+    It is a header if it carries a page number next to uppercase text, or reads as the book title or as
+    the chapter title (extra_titles: the right-hand page repeats the chapter title, and its page number
+    may be misread by the scan as a letter, so no number is found)."""
     i = 0
     while i < len(lines) and not lines[i].strip():
         i += 1
-    j = i
-    while j < len(lines) and lines[j].strip():
+    if i >= len(lines):
+        return None
+    j = i + 1
+    while j < len(lines) and lines[j].strip() and j - i < 3 and _mostly_upper(lines[j]):
         j += 1
     zone = lines[i:j]
-    if not zone:
-        return None
     number, rest = None, []
     for ln in zone:
         m = _NUM_LEAD.match(ln) or None
@@ -46,12 +54,11 @@ def header_zone(lines, title):
             continue
         rest.append(ln)
     text = " ".join(rest)
-    letters = [c for c in text if c.isalpha()]
-    mostly_upper = (not letters) or sum(c.isupper() for c in letters) / len(letters) > 0.7
-    if number is not None and mostly_upper and len(zone) <= 3:
+    if number is not None and _mostly_upper(text) and len(zone) <= 3:
         return i, j, number
-    if title and similar(text, title) >= 0.9:
-        return i, j, None
+    for t in (title,) + tuple(extra_titles):
+        if t and similar(text, t) >= 0.9:
+            return i, j, None
     return None