fix: nur eingebundene Stylesheets auswerten (QA3-01)

Bisher wirkten alle CSS-Dateien des Manifests, auch mitgelieferte Layouts
anderer Buchreihen. Im C-Buch (21 CSS, nur main.css verlinkt) wurden fette
Buchstaben zu Inline-Code und Registerbuchstaben zu <mark>; eine fremde
display:none-Regel konnte Text löschen.

_load_stylesheet wertet jetzt nur per <link rel="stylesheet"> eingebundene
Stylesheets (transitiv inkl. @import) und <style>-Elemente aus; nicht
eingebundene meldet die Info css-unlinked.

Test-Builder: Doc.css, Testdokumente verlinken ihr CSS wie echte EPUBs
(QA3-02). Regressionstests in tests/test_styles.py.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -3,6 +3,7 @@
 from __future__ import annotations
 
 import json
+import posixpath
 import sys
 import zipfile
 from dataclasses import dataclass, field
@@ -25,7 +26,7 @@ SVG = b'<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10"><rect wid
 XHTML = """<?xml version="1.0" encoding="utf-8"?>
 <!DOCTYPE html>
 <html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" lang="de">
-<head><title>{title}</title></head>
+<head><title>{title}</title>{head}</head>
 <body>{body}</body>
 </html>"""
 
@@ -36,6 +37,7 @@ class Doc:
     body: str
     title: str = ""
     linear: bool = True
+    css: list[str] | None = None  # linked stylesheets (OEBPS-relative) – default: every .css in Book.media
 
 
 @dataclass
@@ -90,7 +92,7 @@ def build_epub(path: Path, book: Book) -> Path:
   <spine{' toc="ncx"' if book.ncx else ''}>{''.join(spine)}{book.spine_extra}</spine>
 </package>"""
     nav_xhtml = XHTML.format(
-        title="Inhalt",
+        title="Inhalt", head="",
         body='<nav epub:type="toc"><ol>' + "".join(f'<li><a href="{h}">{lbl}</a></li>' for lbl, h in nav_entries) + "</ol></nav>",
     )
     ncx = (
@@ -114,7 +116,10 @@ def build_epub(path: Path, book: Book) -> Path:
         if book.ncx:
             z.writestr("OEBPS/toc.ncx", ncx)
         for d in book.docs:
-            z.writestr(f"OEBPS/{d.name}", XHTML.format(title=d.title, body=d.body))
+            css = d.css if d.css is not None else [n for n in book.media if n.lower().endswith(".css")]
+            head = "".join(f'<link rel="stylesheet" type="text/css" href="{posixpath.relpath(c, posixpath.dirname(d.name) or ".")}"/>'
+                           for c in css)
+            z.writestr(f"OEBPS/{d.name}", XHTML.format(title=d.title, head=head, body=d.body))
         for name, data in book.media.items():
             z.writestr(f"OEBPS/{name}", data)
         for name, data in book.extra_entries.items():