fix: Hanging indent (negative text-indent) and em-unit sizing (#1229)

## Summary

* **What is the goal of this PR?** Fixing two independent CSS rendering
bugs combined to make hanging-indent list styles
(e.g. margin-left:3em; text-indent:-1em) render incorrectly:

* **What changes are included?**
  1. Negative text-indent was silently ignored

Three guards in ParsedText.cpp (computeLineBreaks,
computeHyphenatedLineBreaks,
extractLine) conditioned firstLineIndent on blockStyle.textIndent > 0,
so any
negative value collapsed to zero. Additionally, wordXpos was uint16_t,
which
cannot represent negative offsets — a cast of e.g. −18 would wrap to
65518 and
      render the word far off-screen.

   2. extraParagraphSpacing suppressed hanging indents

Even after removing the > 0 guard, the existing !extraParagraphSpacing
condition
would still suppress all text-indent when that setting is on (its
default). Positive
text-indent is a decorative paragraph indent that the user can
reasonably replace with
vertical spacing — negative text-indent is structural (it positions the
list marker)
      and must always apply.

   3. em unit was calibrated against line height, not font size

emSize was computed as getLineHeight() * lineCompression (the full line
advance).
CSS em units are defined relative to the font-size, which corresponds to
the
ascender height — not the line height. Using line height makes every
em-based
margin/indent ~20–30% wider than a browser would render it, and is
especially
noticeable for CSS that uses font-size: small (which we do not
implement).

## Additional Context

Test case
```
.lsl1 { margin-left: 3em; text-indent: -1em; }

<div class="lsl1">• First list item that wraps across lines</div>
<div class="lsl1">• Short item</div>
```
Before: all lines of all items started at 3 em from the left edge
(indent ignored).

After: the bullet marker hangs at 2 em; continuation lines align at 3
em.

<img width="240" alt="before"
src="https://github.com/user-attachments/assets/9dcbf3e0-fcd9-4af8-b451-a90ba4d2fb75"
/>
<img width="240" alt="after"
src="https://github.com/user-attachments/assets/1ffdcf56-a180-4267-9590-c60d7ac44707"
/>

---

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Did you use AI tools to help write this code? _**YES**_

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

@@ -278,8 +278,7 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
 
                 int displayWidth = 0;
                 int displayHeight = 0;
-                const float emSize =
-                    static_cast<float>(self->renderer.getLineHeight(self->fontId)) * self->lineCompression;
+                const float emSize = static_cast<float>(self->renderer.getFontAscenderSize(self->fontId));
                 CssStyle imgStyle = self->cssParser ? self->cssParser->resolveStyle("img", classAttr) : CssStyle{};
                 // Merge inline style (e.g. style="height: 2em") so it overrides stylesheet rules
                 if (!styleAttr.empty()) {
@@ -505,7 +504,7 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
     }
   }
 
-  const float emSize = static_cast<float>(self->renderer.getLineHeight(self->fontId)) * self->lineCompression;
+  const float emSize = static_cast<float>(self->renderer.getFontAscenderSize(self->fontId));
   const auto userAlignmentBlockStyle = BlockStyle::fromCssStyle(
       cssStyle, emSize, static_cast<CssTextAlign>(self->paragraphAlignment), self->viewportWidth);