Name the highest spec_version Borea implements, and expose the comparison

[Maximilian-Nesslauer]

ModMetadata and ModVersionMetadata validate spec_version only at the bottom: both throw below 1 and accept anything above.
The storage mappers additionally reject 0 as the pre-model shape.
Nothing compares the value to a ceiling, so a document stamped spec_version = 7 constructs cleanly and flows through search, storage and CompositeModRepository as if fully understood.

RFC 0031: "Metadata a client cannot interpret, including a spec_version newer than it implements, renders as an entry in an unknown state; it is never silently dropped."

• One constant in Borea.Core naming the highest spec version Borea implements, which is 1.
• A non-throwing way for a caller to ask whether a document is above it. Follow MetadataEnumMapper, where every parse falls through to an Unknown member rather than throwing. Do not follow ModMetadataMapper.FromDto, which throws for the missing-spec-version case and is the opposite of what is wanted here.
• Replace the two hardcoded specVersion: 1 literals in SpaceDockModRepository with the constant.
• Tests for below, equal and above. The existing tests only ever pass 1 and never assert the below-1 throw, so that is new coverage too.

Related: #31 adds spec_version to ModPackMetadata, which carries none today. Whichever lands second consumes the same constant.