Make instance fake work for KSA

[Maximilian-Nesslauer]

What

instance fake --game KSA always failed, for the two stacked reasons described in #30. This makes it work end to end; it is the client-side prerequisite for the xKAN-meta_testing step of KSP-CKAN#4457 (the metadata tester verifies mods by installing them into a fake instance).

• FakeInstance and the CmdLine fake path normalize the requested version (build counter pinned to 0, matching how every KSA version is stored since Ignore KSA's non-monotonic build counter for version ordering #14), so the raw in-game version string (2026.6.9.4750) is accepted.
• FakeInstance writes a minimal Content/Versions/v<year>.<month>.X.<revision>.json with the build field, which is the only on-disk version source KsaBuildVersionProvider reads (KSP2 falls back to its known-versions list, KSA does not). The faked instance now has a detectable version and passes AddInstance. The directory, file name, and body helpers live next to the provider so reader and writer share the contract, mirroring the KspBuildIdVersionProvider.buildIDfilenames precedent.

Hardening on top, from a code-review pass over the change:

• The full KSA version including the revision is validated against the build map. Checking WithoutBuild (the pre-existing gate) would have degenerated the check to year.month for KSA and accepted revisions that never shipped.
• The CLI rejects an unknown fully defined KSA version outright instead of letting the version selection dialog silently substitute a different known revision of the same month. For KSA the revision is the real game version, unlike the build-metadata 4th component of KSP.
• The KSP-only --MakingHistory/--BreakingGround options are rejected for other games instead of creating bogus KSP DLC folders inside the faked instance (previously reachable for KSP2 as well).

Testing

• New Core tests: the raw and the normalized version string both register a valid instance with the normalized version and the version file; an unknown year.month and an unknown revision within a known month throw BadGameVersionKraken.
• New CmdLine end-to-end tests: raw version works, unknown revision fails, DLC options for KSA fail.
• Full net10.0 test suite green; net481 builds clean.

Closes #30.