[Kilombino]

Follow-up to #1, as you suggested: refuse a v2 header that commits to a height below the BLAKE2b activation, as Knots does in CheckBlockHeader (bad-version-sha256d).

• CheckHeaderV2Sanity(header, params) is the context-free part of Knots' CheckBlockHeader for v2 headers: the committed height must be a BLAKE2b height, and a network without BLAKE2b configured takes no v2 header (ErrUnsupportedProofOfWork, as in checkHeaderV2Context). It needs only the header, so it covers the case where the real height isn't known yet (an orphan block); checkHeaderV2Context still checks the committed height against the real one once it is.
• Called next to checkBlockSanity in ProcessBlock and CheckConnectBlockTemplate, and in verifychain. CheckBlockHeaderSanity keeps its signature (neutrino calls it with powLimit only); the new function is exported so neutrino's context-free check can call it too, which I can send once this is tagged.
• TestCheckHeaderV2Sanity: the real activation header (961640) is accepted, the same header committing to 961639 is refused with ErrBadHeaderV2, a classic header passes, and a network without BLAKE2b refuses the v2 one.

go test ./... passes except TestHelp (getblock/getblockheader verbose results have no help for mm_rhs), which fails the same on blake2b without this change.

[paulscode]

Thanks, merged. It matches Knots' bad-version-sha256d rule and already sat on v0.25.1-blake2b.7. Tests pass for blockchain, chaincfg, txscript and mempool, and the new test fails when the rule is disabled. A block that passes the context check always passes this rule, so nothing valid is refused, including the block at 961640, testnet4 and the networks without BLAKE2b, and invalid orphans are now dropped earlier. TestHelp fails the same way on the base (the mm_rhs help strings).

Two nits if you send the neutrino follow-up: no test covers the call sites (removing the call in process.go fails nothing), and the comment in checkProofOfWork still says the height question is answered only in the context check.