Regtest difficulty retargets, but zcashd regtest expects fixed powLimit (sidecar rejects headers as bad-diffbits)

[arya2]

expected_difficulty_threshold has no Regtest case — only the testnet minimum-difficulty branch, else full retargeting:

zakura/zakura-state/src/service/check/difficulty.rs

Lines 188 to 203 in 7880676

	pub fn expected_difficulty_threshold(&self) -> CompactDifficulty {
	if NetworkUpgrade::is_testnet_min_difficulty_block(
	&self.network,
	self.candidate_height,
	self.candidate_time,
	*self.relevant_times.first(),
	) {
	assert!(
	self.network.is_a_test_network(),
	"invalid network: the minimum difficulty rule only applies on test networks"
	);
	self.network.target_difficulty_limit().to_compact()
	} else {
	self.threshold_bits()
	}
	}

zcashd regtest uses fixed powLimit with no retargeting (fPowNoRetargeting). Once the difficulty-averaging window fills (~17 blocks), the nBits of blocks mined on a zakura regtest chain diverge from powLimit, and a following zcashd rejects the headers as bad-diffbits — a permanent fork at that height. -regtestacceptunvalidatedpow does not help: it skips equihash/block-hash checks, not the contextual nBits check.

Regtest-only; no mainnet impact. It does prevent any regtest test that mines more than ~17 blocks (e.g. coinbase-maturity flows, which need 100+) from working with a zcashd sidecar.

Reproduce: mine ~20 regtest blocks on zakurad with a zcashd sidecar following; the sidecar rejects the first post-window header with bad-diffbits and stops following.

Fix reference: fixed upstream in Zebra (ZcashFoundation/zebra#10952, commit c7691893e): pin every Regtest block to the target difficulty limit, matching zcashd's fPowNoRetargeting.