Add ConvertibleTo[Format] annotation

[tclose]

• ConvertibleTo[X] annotates "any format that can be converted to X"; it evaluates to X.convertible_from().
• convertible_from() now returns Annotated[<union>, ConvertibleToMarker(X)], so the target format can be recovered from a hint with ConvertibleToMarker.from_hint(). Breaking: callers comparing against the bare union need to strip the annotation.
• The union is sorted from most to least specific (subclasses before base classes) instead of by name.
• to_mime/from_mime round-trip the hint as <namespace>/<format>+convertible-to (mime-like only), so the union is re-resolved from the converters installed where the string is read.
• to_mime now passes official through for optional types.
• LoadedMarker and ConvertibleToMarker share a new FormatMarker base.

[codecov]

Codecov Report

❌ Patch coverage is 84.37500% with 10 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
fileformats/core/convertible.py	74.07%	4 Missing and 3 partials ⚠️
fileformats/core/identification.py	76.92%	1 Missing and 2 partials ⚠️

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