Use BIP9 activation for ASERT

[NyanCatTW1]

Goal

Ensure ASERT only activates after miners have explicitly signaled readiness,
replacing the hardcoded timestamp activation with BIP9 versionbits consensus.
This guarantees the DAA switch happens only when a supermajority (95% on
mainnet) of hashrate has upgraded and agreed to the change.

Summary

• BIP9 activation for ASERT: Replace asertActivationTime timestamp gate
  with a proper DEPLOYMENT_ASERT versionbits deployment (bit 24). Mainnet
  signaling window runs 2026-03-01 to 2027-03-01 with period=672 and
  threshold=639 (95%). Testnet/signet use ALWAYS_ACTIVE; regtest uses real BIP9
  (nStartTime=0, period=32) so the full state machine is exercisable in tests.
• Unified anchor discovery: GetASERTAnchorBlock now uses
  StateSinceHeight to derive the anchor for all networks — block 1 for
  ALWAYS_ACTIVE, last block of the LOCKED_IN period for real BIP9 — eliminating
  the old asertActivationTime == 0 special case.

Details

Consensus params:

• consensus/params.h — remove asertActivationTime, add DEPLOYMENT_ASERT
  to DeploymentPos enum
• deploymentinfo.cpp — register deployment name and gbt_force = true
• chainparams.cpp — configure DEPLOYMENT_ASERT per network, fix TESTDUMMY
  min_activation_height (1 → 0) to satisfy period-boundary invariant

Activation logic:

• pow.cpp — IsASERTEnabled delegates to DeploymentActiveAfter;
  GetASERTAnchorBlock replaced timestamp walk-back with StateSinceHeight
  query; GetNextFACTORASERTWorkRequired uses a single GetASERTAnchorBlock
  call for all networks

Tests:

• pow_tests.cpp — anchor tests now use RegtestTestingSetup with a
  BuildBIP9ActivatedChain helper that constructs a chain with explicit
  version-bit signaling (blocks 32-63), exercising the full
  DEFINED → STARTED → LOCKED_IN → ACTIVE state machine. Covers anchor
  discovery, cache invalidation on reorg, fast blocks, and slow blocks
• versionbits_tests.cpp — check_computeblockversion respects per-deployment
  period overrides instead of assuming global nMinerConfirmationWindow

Smoke tests performed

• IBD still works
• Testnet run looks good: 20260415-testnet.txt