Re-enable proof-of-work validation and add chain checkpoints

pow.cpp: CheckProofOfWork() had the hash-vs-target comparison commented
out, so it only ran the range check and accepted any hash for the
claimed nBits. Re-enabled it. Verified safe by a full -reindex of the
live chain (height ~11700) with the check enabled: every block,
including the PoW range 1-10000 and all PoS blocks, validates with 0
errors, confirming the historical chain has valid proof-of-work.

chainparams.cpp: the mainnet checkpoint map only contained genesis.
Added checkpoints at blocks 1, 5000, 10000 (PoW->PoS boundary) and
11000 (all from the canonical chain) and updated CCheckpointData, for
reorg / long-range-attack protection.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: NateIsern

src/chainparams.cpp

@@ -331,12 +331,17 @@ static void convertSeed6(std::vector<CAddress>& vSeedsOut, const SeedSpec6* data
 //    timestamp before)
 // + Contains no strange transactions
 static Checkpoints::MapCheckpoints mapCheckpoints =
-    boost::assign::map_list_of(0, uint256("0x00000232cb134567cf85cd65748714df75d72fe4ce71cf77d3c3f8a9a1a576e6"));
+    boost::assign::map_list_of
+    (0, uint256("0x00000232cb134567cf85cd65748714df75d72fe4ce71cf77d3c3f8a9a1a576e6"))
+    (1, uint256("0x00000c7be1164f34a243d233c94ec23e6fdc76813ac94b720a624d4ed52c9f0c"))
+    (5000, uint256("0x000000001c5f95c383855c71fa5ee106efd0ce9b6c93198b55cd00f47c84126f"))
+    (10000, uint256("0x00000000a840c1a7ca180f542fa26beed769bf0a91ba60c65b1d3d60911308c0")) // last PoW block (PoW->PoS boundary)
+    (11000, uint256("0x9f6d23e28485c926d63e6a249f9983472f1e7bd467db56864a575891819ebd84"));
 static const Checkpoints::CCheckpointData mainCheckpointData = {
     &mapCheckpoints,
-    1744156800, // April 9, 2026 00:00:00 UTC
-    0,
-    0
+    1777017373, // block 10000 time (2026-04-24)
+    10005,      // total tx up to last checkpoint (approx)
+    500         // estimated tx per day after checkpoint
 };
 
 static Checkpoints::MapCheckpoints mapCheckpointsTestnet =

src/pow.cpp

@@ -123,8 +123,8 @@ bool CheckProofOfWork(uint256 hash, unsigned int nBits)
         return error("CheckProofOfWork() : nBits below minimum work");
 
     // Check proof of work matches claimed amount
-    ///////////if (hash > bnTarget)
-        //////////////return error("CheckProofOfWork() : hash doesn't match nBits");
+    if (hash > bnTarget)
+        return error("CheckProofOfWork() : hash doesn't match nBits");
 
     return true;
 }