feat(testnet): short PoW phase + fast stake age so testnet is lively

Builds on the testnet block-1 premine fix. Tunes testnet-only params so a fresh
chain reaches live PoS in minutes instead of hours:
- CTestNetParams nLastPOWBlock 200 -> 25 (mine the PoW phase in minutes).
- nStakeMinAge 7200s -> 60s on testnet only (PoS starts ~1 min after mining,
  not 2 hours). Mainnet keeps 7200s.

Mainnet consensus is unchanged; this binary ships only as a *-testnet prerelease.

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

Author: NateIsern

src/chainparams.cpp

@@ -472,7 +472,9 @@ class CTestNetParams : public CMainParams
         nMinerThreads = 0;
         nTargetTimespan = 1 * 60; // faircoin: 1 day
         nTargetSpacing = 1 * 60;  // faircoin: 1 minute
-        nLastPOWBlock = 200;
+        // Short PoW phase so a fresh testnet can be mined to PoS in minutes
+        // (mainnet is 10000). After this height blocks must be PoS.
+        nLastPOWBlock = 25;
         nMaturity = 15;
         nMasternodeCountDrift = 4;
         nModifierUpdateBlock = 51197; //approx Mon, 17 Apr 2017 04:00:00 GMT

src/init.cpp

@@ -691,6 +691,14 @@ bool AppInit2(boost::thread_group& threadGroup)
     fLogTimestamps = GetBoolArg("-logtimestamps", true);
     fLogIPs = GetBoolArg("-logips", false);
 
+    // Testnet: shorten the minimum stake age so a freshly-bootstrapped chain
+    // starts producing PoS blocks within ~1 minute of the PoW phase instead of
+    // 2 hours. Mainnet keeps the default nStakeMinAge (7200s).
+    if (Params().NetworkID() == CBaseChainParams::TESTNET) {
+        nStakeMinAge = 60;
+        LogPrintf("AppInit2 : testnet -> nStakeMinAge=%u\n", nStakeMinAge);
+    }
+
     if (mapArgs.count("-bind") || mapArgs.count("-whitebind")) {
         // when specifying an explicit binding address, you want to listen on it
         // even when -connect or -proxy is specified