Evaluate compatibility per RFC 0017 and the os list per RFC 0031 #39

ModVersionMetadata carries GameMinRevision, GameMaxRevision and Os.

RFC 0017 defines four states:

Incompatible is the only state that blocks.
This classifies only; what a client does with Untested and Unknown is a UI decision and not part of this issue.

Shape, in Borea.Core:

• enum GameCompatibility { Compatible, Untested, Incompatible, Unknown }
• Evaluate(int? minRevision, int? maxRevision, GameVersion installed), plus an overload taking a ModVersionMetadata.
• Compare installed.Revision against the bound integers directly.

On Unknown: ModVersionMetadata requires gameMin and a non-nullable GameMinRevision, so the release overload can never return Unknown and there is no "no bounds at all" case for a release. That is expected, not a gap to fix here. Unknown exists for the listing side, where SpaceDockModRepository maps an unparseable game version to GameMin = "unknown". The nullable parameter is what makes the state reachable.

The os list, from the same [compatibility] table. RFC 0031: "A platform outside the stated os list warns and lets the user proceed: the list states what the author knows works, not everything that can." Advisory, never blocking.

• The vocabulary is windows, linux, macos; the RFC adds "New values arrive by RFC".
• The check takes the target platform as a parameter and never reads the running OS, so it stays testable on the three-OS CI matrix.
• Absent means no known restriction. Treat an empty list the same way, and say in a comment that this is our choice: the RFC defines only absent, and the domain deliberately keeps empty distinct from absent.
• Compare case-insensitively as client leniency; the RFC states no comparison rule for os.
• An unrecognised value must not make the list unusable and must not be silently dropped: keep evaluating the recognised entries and report the unrecognised ones, so a caller can say the listing knows a platform this build does not.
• Give the Unknown member a doc comment saying it means no usable compatibility data, because ModStatus.Unknown and ReleaseStatus.Unknown already use the name for "a value this client version does not know".