[Breaking change] Load platform fonts fallback to avoid squares instead of glyphs - #49

The matrix only covered the three example apps, so nothing under `test/` ever
ran on CI. Format and analyze are left out of this job for now: the package
sources still carry pre-existing analyzer infos.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

A font bundled by a dependency is exposed by the font manifest as
`packages/my_theme/Roboto`, and `loadFonts()` only registered it under that
name. A widget styled with `TextStyle(fontFamily: 'Roboto', package:
'my_theme')` — or a theme defaulting to that family — asks for the bare
`Roboto` instead, so no font was found and the text rendered as placeholder
blocks in the goldens.

Register both names. `loadFonts()` now returns the families it registered,
which is also what lets the tests assert on it without shared state.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

The families a widget gets when it specifies no font — `Roboto` on Android,
`CupertinoSystemText` on iOS, `Segoe UI` on Windows — are declared in no
`pubspec.yaml`: they either ship with the Flutter SDK or belong to the host OS.
`loadFonts()` could not find them in the font manifest, so a `DatePickerDialog`
or a `CupertinoButton` rendered as placeholder blocks in the goldens.

The list is gathered from `Typography` and `CupertinoTextThemeData` instead of
being hardcoded, so it follows the SDK. The families the SDK ships are loaded
from its font cache, found through `FLUTTER_ROOT` or by walking up from the
test executable. The ones only the OS can provide are aliased to the typeface
the SDK ships: the glyphs then differ from a real device, but the goldens stay
readable. Opt out with `setLoadPlatformFallbackFonts(false)`.

`FontLoadingPolicy` holds what is registered on top of the manifest, and with
it the arbitration introduced here: a dependency font does not claim the bare
name of a platform default the SDK ships a font for. It usually bundles a
single weight of such a family while the SDK ships all of them, and on a device
it would not shadow the platform font either.

The SDK font files are sorted before being registered: a directory listing
comes back in file system order, which differs between APFS, ext4 and NTFS,
and goldens must not depend on the machine that generated them.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>