fix(fonts): resolve relative asset paths from the install root, not the cwd

resolve_asset_path checked os.path.exists(relative_path) unconditionally,
so a relative asset path was still resolved against the process cwd first
-- exactly the dependency this module exists to remove. An unrelated
working directory that happens to contain assets/fonts/4x6-font.ttf (a
stale checkout, a copied assets folder, another project) would shadow the
real bundled font instead of the install root ever being consulted.

Only an absolute path is now returned as-is; a relative path always
resolves against _INSTALL_ROOT first, matching the docstring's stated
contract. FontManager._resolve_asset_path delegates to this function, so
it's covered by the same fix.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/common/font_layout.py

@@ -77,7 +77,7 @@ def resolve_asset_path(relative_path: str) -> str:
     file written without ``WorkingDirectory``) silently loses every font and
     degrades to PIL's default face.
     """
-    if os.path.exists(relative_path):
+    if os.path.isabs(relative_path) and os.path.exists(relative_path):
         return relative_path
     candidate = _INSTALL_ROOT / relative_path
     if candidate.exists():