Use any installed Noto font when named families lack glyphs (closes #1722)

SystemFontProvider.NOTO_FONT_PATTERNS enumerates about two dozen Noto
families by name, and MultiFontManager only ever asked for those. Any
script outside that list fell through to the glyphless Occulta fallback
even when the correct font was installed, which is the common case on
macOS: it ships around a hundred script-specific Noto faces in
/System/Library/Fonts/Supplemental, almost none of which we knew how to
ask for. The reporter had the fonts and still got told to install them.

Add an optional GlyphSearchingFontProvider protocol (find_font_with_glyphs)
implemented by SystemFontProvider, BuiltinFontProvider and
ChainedFontProvider, and a new selection phase that uses it once the named
families fail. The system scan enumerates every Noto face, reusing the
existing variable-font/-Regular/-VF filename classification, and keeps
only the font it selects so a full scan does not retain every font file on
the system. Fonts found this way are remembered and retried by name for
later words. Capability detection is isinstance-based so third-party
providers keep working unchanged.

The warning itself was also unactionable: it named neither the characters
nor the script, which is why the reporter had to ask which package to
install. It now identifies what it could not render, e.g.
'Ꮳ' U+13E3 CHEROKEE LETTER TSA. A word mixing scripts that no single font
covers now gets a distinct message saying that installing fonts will not
help, rather than sending the user after fonts they already have.

Finally, the documented macOS install command was wrong: `brew install
font-noto` does not exist, since Homebrew has no single Noto package, only
a cask per family. The Fedora package name was also corrected, as
google-noto-fonts-common ships no actual fonts.

docs/installation.md

@@ -663,9 +663,25 @@ provides text shaping for proper multilingual support. These replace the
 legacy hOCR-based renderer. Install with: `pip install fpdf2 uharfbuzz`
 
 **fonts-noto** (or an equivalent comprehensive font package) is recommended
-for proper text rendering, especially for non-Latin scripts. On Debian/Ubuntu:
-`apt install fonts-noto`. On Fedora: `dnf install google-noto-fonts-common`.
-On macOS with Homebrew: `brew install font-noto`.
+for proper text rendering, especially for non-Latin scripts. OCRmyPDF bundles
+a Latin font only, and discovers the rest from the fonts installed on your
+system.
+
+- Debian/Ubuntu: `apt install fonts-noto`
+- Fedora: `dnf install google-noto-fonts-all`
+- macOS with Homebrew: Homebrew has no single Noto package; each family is a
+  separate cask. Install at least
+  `brew install --cask font-noto-sans font-noto-serif`, plus a cask per
+  additional script you OCR, for example
+  `brew install --cask font-noto-sans-arabic font-noto-sans-cjk`. Run
+  `brew search font-noto` to list them all.
+
+If OCRmyPDF warns that no installed font has glyphs for some of the text, the
+message names the characters it could not render, for example
+`'Ꮳ' U+13E3 CHEROKEE LETTER TSA`. Install the Noto font for that script — here,
+`fonts-noto-core` on Debian or `font-noto-sans-cherokee` on Homebrew. The text
+layer remains searchable and copyable either way; only its appearance when
+highlighted in a PDF viewer is affected.
 
 **pypdfium2**, if present, provides fast PDF page rasterization using
 the pdfium library (the same library used by Google Chrome). It is

docs/maintainers.md

@@ -47,7 +47,9 @@ OCRmyPDF has the following runtime dependencies:
 **For text rendering** (expressing OCR results in PDF):
 - `fpdf2` (Python package) - Required for text layer rendering
 - `uharfbuzz` (Python package) - Required for text layer rendering
-- `font-noto` (system package) - Recommended for text layer rendering
+- Noto fonts (system package) - Recommended for text layer rendering.
+  `fonts-noto` on Debian/Ubuntu, `google-noto-fonts-all` on Fedora; Homebrew
+  has no single Noto package, only per-family casks such as `font-noto-sans`.
 
 **Other dependencies**:
 - `unpaper` (system binary) - Optional, enables `--clean` and `--clean-final`

docs/releasenotes/version17.md

@@ -3,6 +3,27 @@
 
 # v17
 
+## v17.9.0
+
+- OCRmyPDF now uses any Noto font installed on the system, not just the two
+  dozen script families it knows by name ({issue}`1722`). Previously a document
+  in, say, Cherokee or Vai was rendered with the glyphless fallback font even
+  though the matching font was installed — a common situation on macOS, which
+  ships around a hundred script-specific Noto faces. When the named fonts
+  cannot cover a word, OCRmyPDF now searches the installed fonts for one that
+  can.
+- The "no installed font has glyphs" warning now names the characters it could
+  not render, with their codepoints and Unicode names, so it is clear which
+  font to install. Text that mixes scripts no single font covers is now
+  reported as such, instead of advising the user to install fonts they may
+  already have.
+- Fixed the macOS font installation instructions, which recommended a Homebrew
+  package (`font-noto`) that does not exist ({issue}`1722`). Homebrew has no
+  single Noto package; each family is a separate cask. The Fedora package name
+  was also corrected to `google-noto-fonts-all`.
+- Font providers may now implement the optional `GlyphSearchingFontProvider`
+  protocol to participate in coverage-based font search.
+
 ## v17.8.1
 
 - Improved the `--tesseract-pagesegmode` help text to point to