fix(text): constant word space for ragged alignment, exact justification (S13)

LeftAlignmentHandler spread each line's residual space across its word gaps,
clamped to max_spacing. A line whose residual divided to under max_spacing was
stretched flush, one that exceeded it was not, so left-aligned text was
justified sometimes, by a different amount per line - which reads as a wobbling
right edge rather than as ragged-right. Centre/right did the same, and computed
their start position from a different spacing than the one they returned, so
centred lines were not centred.

Ragged alignments now use a constant word space - the font's own space advance,
clamped to the style's bounds - and report overflow instead of tightening, so
line breaking decides what fits rather than rendering squeezing it.

Justification kept two further defects:

  - the final line of a paragraph was stretched across the measure, so a
    three-word tail was spread edge to edge. Line now carries is_paragraph_end,
    set on the line holding the last word, and renders flush left. A paragraph
    continued on the next page is not marked, so it stays justified.

  - gaps were floored per gap with a truncated remainder, discarding the
    fractional part of both. Lines stopped one or two pixels short, differently
    each time. Distributing by cumulative rounding makes the gaps sum to the
    residual exactly; advance ends now land identically on every line.

Alignment is configurable rather than hardcoded: PageStyle.default_alignment,
defaulting to JUSTIFY for body text. text_align on abstract and concrete styles
defaults to None meaning "unspecified", so HTML without text-align inherits the
page default while explicit CSS still wins. Headings are never justified.

pyWebLayout/layout/document_layouter.py

@@ -8,7 +8,7 @@ from pyWebLayout.concrete.image import RenderableImage
 from pyWebLayout.concrete.functional import ButtonText, FormFieldText
 from pyWebLayout.concrete.table import TableRenderer, TableStyle
 from pyWebLayout.abstract import Paragraph, Word
-from pyWebLayout.abstract.block import Image as AbstractImage, PageBreak, Table
+from pyWebLayout.abstract.block import Image as AbstractImage, Heading, PageBreak, Table
 from pyWebLayout.abstract.functional import Button, Form, FormField
 from pyWebLayout.style.concrete_style import ConcreteStyleRegistry, RenderingContext, StyleResolver
 from pyWebLayout.style import Font, Alignment
@@ -51,6 +51,15 @@ def paragraph_layouter(paragraph: Paragraph,
     # We need to get word spacing constraints from the Font's abstract style if available
     # For now, use reasonable defaults based on font size
 
+    # Alignment for text that does not specify its own. Headings are never
+    # justified - stretching a two-word title across the measure is always wrong -
+    # so they fall back to flush left.
+    default_alignment = getattr(page.style, 'default_alignment', None)
+    if not isinstance(default_alignment, Alignment):
+        default_alignment = Alignment.JUSTIFY
+    if isinstance(paragraph, Heading):
+        default_alignment = Alignment.LEFT
+
     if isinstance(paragraph.style, Font):
         # paragraph.style is already a Font (concrete style)
         font = paragraph.style
@@ -59,7 +68,7 @@ def paragraph_layouter(paragraph: Paragraph,
         min_spacing = float(font.font_size) * 0.25  # 25% of font size
         max_spacing = float(font.font_size) * 0.5   # 50% of font size
         word_spacing_constraints = (int(min_spacing), int(max_spacing))
-        text_align = Alignment.LEFT  # Default alignment
+        text_align = default_alignment
     else:
         # paragraph.style is an AbstractStyle, resolve it
         # Ensure font_size is an int (it could be a FontSize enum)
@@ -79,7 +88,8 @@ def paragraph_layouter(paragraph: Paragraph,
             int(concrete_style.word_spacing_min),
             int(concrete_style.word_spacing_max)
         )
-        text_align = concrete_style.text_align
+        # text_align is None when the source did not specify one.
+        text_align = concrete_style.text_align or default_alignment
 
     # Apply page-level word spacing override if specified
     if hasattr(
@@ -260,7 +270,13 @@ def paragraph_layouter(paragraph: Paragraph,
             else:
                 current_pretext = overflow_text  # May be None or hyphenated remainder
 
-    # All words processed successfully
+    # All words processed successfully. The line holding the final word is the
+    # end of the paragraph, so it is rendered at its natural width rather than
+    # justified to the full column. A paragraph continued on the next page does
+    # not reach here, so its lines stay justified - which is correct.
+    if current_line is not None:
+        current_line.is_paragraph_end = True
+
     return True, None, None