Fix horizontal box-drawing alignment with textLength attribute

Horizontal line characters (─━═) render narrower than the intended
character width in most fonts, causing gaps when followed by other
characters. Now using textLength + lengthAdjust='spacing' to force
horizontal box-drawing spans to occupy their correct width.

- Added _is_all_horizontal_box_drawing() helper
- Added textLength attribute for horizontal line spans > 1 char
- Added comprehensive tests for new functionality
- svg_exporter.py now has 100% test coverage

Version bump to 0.3.8

tests/test_svg_exporter.py

@@ -10,6 +10,7 @@ from textual_webterm.svg_exporter import (
     _build_row_spans,
     _color_to_hex,
     _escape_xml,
+    _is_all_horizontal_box_drawing,
     _is_box_drawing_vertical_or_corner,
     _should_break_span,
     render_terminal_svg,
@@ -387,6 +388,30 @@ class TestBoxDrawingHelpers:
         """Horizontal lines can merge with each other."""
         assert _should_break_span("─", "─") is False
         assert _should_break_span("━", "━") is False
+
+    def test_is_all_horizontal_box_drawing_empty(self) -> None:
+        """Empty string returns False."""
+        assert _is_all_horizontal_box_drawing("") is False
+
+    def test_is_all_horizontal_box_drawing_normal_text(self) -> None:
+        """Normal text returns False."""
+        assert _is_all_horizontal_box_drawing("Hello") is False
+        assert _is_all_horizontal_box_drawing("ABC") is False
+
+    def test_is_all_horizontal_box_drawing_horizontal_lines(self) -> None:
+        """Horizontal box chars return True."""
+        assert _is_all_horizontal_box_drawing("─") is True
+        assert _is_all_horizontal_box_drawing("───") is True
+        assert _is_all_horizontal_box_drawing("━━━") is True
+        assert _is_all_horizontal_box_drawing("═══") is True
+
+    def test_is_all_horizontal_box_drawing_mixed(self) -> None:
+        """Mixed content returns False."""
+        assert _is_all_horizontal_box_drawing("─A─") is False
+        assert _is_all_horizontal_box_drawing("│──") is False  # vertical at start
+
+
+class TestRenderTerminalSvg:
     """Tests for render_terminal_svg function."""
 
     def _char(
@@ -797,6 +822,20 @@ class TestEdgeCases:
         assert "─" in svg
         assert "┐" in svg
 
+    def test_horizontal_lines_use_textlength(self) -> None:
+        """Horizontal line spans use textLength for correct width."""
+        buffer = [[
+            self._char("╭"),
+            self._char("─"),
+            self._char("─"),
+            self._char("─"),
+            self._char("╮"),
+        ]]
+        svg = render_terminal_svg(buffer, width=5, height=1)
+        # Horizontal lines should have textLength attribute
+        assert 'textLength="24.0"' in svg
+        assert 'lengthAdjust="spacing"' in svg
+
     def test_ansi_bright_colors(self) -> None:
         """All bright ANSI colors render."""
         colors = ["brightred", "brightgreen", "brightyellow",