fix: release RandomX pre-verify cache at shutdown; tune verify-threads help + -maxblocksintransit ceiling

Three small cleanups to the parallel RandomX pre-verify + P2P-window features:

- Call RandomXValidatorShutdown() in Shutdown() to release the ~256MB shared RandomX
  verify cache. It was allocated on first use but never freed, leaking at every exit.
  Safe here: threadGroup.interrupt_all() (earlier in Shutdown) stops the pre-verify
  worker, and the release takes g_rxvMutex so it can't race a mid-flight verify.
- Clarify the -randomxverifythreads help: the pool only helps NETWORK sync, not reindex
  (reindex runs with a window of 1, so the pool does nothing there).
- Clamp -maxblocksintransit to the real BLOCK_DOWNLOAD_WINDOW (1024) ceiling instead of a
  misleading 4096. Values above the window are a silent no-op (FindNextBlocksToDownload
  never fetches beyond pindexLastCommonBlock + BLOCK_DOWNLOAD_WINDOW); log when clamping.

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

src/init.cpp

@@ -283,6 +283,7 @@ void Shutdown()
     }
 #endif
     UnregisterAllValidationInterfaces();
+    RandomXValidatorShutdown(); // release the ~256MB shared RandomX pre-verify cache (was leaked at exit)
 #ifdef ENABLE_WALLET
     delete pwalletMain;
     pwalletMain = NULL;
@@ -396,7 +397,7 @@ std::string HelpMessage(HelpMessageMode mode)
     strUsage += HelpMessageOpt("-mempooltxinputlimit=<n>", _("[DEPRECATED/IGNORED] Set the maximum number of transparent inputs in a transaction that the mempool will accept (default: 0 = no limit applied)"));
     strUsage += HelpMessageOpt("-par=<n>", strprintf(_("Set the number of script verification threads (%u to %d, 0 = auto, <0 = leave that many cores free, default: %d)"),
         -(int)boost::thread::hardware_concurrency(), MAX_SCRIPTCHECK_THREADS, DEFAULT_SCRIPTCHECK_THREADS));
-    strUsage += HelpMessageOpt("-randomxverifythreads=<n>", strprintf(_("Number of threads for parallel RandomX PoW pre-verification of post-checkpoint blocks during sync (0 = inline only, max %d, default: same as -par)"), MAX_SCRIPTCHECK_THREADS));
+    strUsage += HelpMessageOpt("-randomxverifythreads=<n>", strprintf(_("Number of threads for parallel RandomX PoW pre-verification of post-checkpoint blocks during network sync; no effect on reindex (0 = inline only, max %d, default: same as -par)"), MAX_SCRIPTCHECK_THREADS));
 #ifndef _WIN32
     strUsage += HelpMessageOpt("-pid=<file>", strprintf(_("Specify pid file (default: %s)"), "hushd.pid"));
 #endif
@@ -1448,8 +1449,12 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     MAX_BLOCKS_IN_TRANSIT_PER_PEER = GetArg("-maxblocksintransit", DEFAULT_MAX_BLOCKS_IN_TRANSIT_PER_PEER);
     if (MAX_BLOCKS_IN_TRANSIT_PER_PEER < 1)
         MAX_BLOCKS_IN_TRANSIT_PER_PEER = 1;
-    else if (MAX_BLOCKS_IN_TRANSIT_PER_PEER > 4096)
+    else if (MAX_BLOCKS_IN_TRANSIT_PER_PEER > (int)BLOCK_DOWNLOAD_WINDOW) {
-        MAX_BLOCKS_IN_TRANSIT_PER_PEER = 4096;
+        // Values above BLOCK_DOWNLOAD_WINDOW are a silent no-op: FindNextBlocksToDownload never fetches
+        // beyond pindexLastCommonBlock + BLOCK_DOWNLOAD_WINDOW, so clamp to the real effective ceiling.
+        LogPrintf("-maxblocksintransit=%d exceeds the effective ceiling BLOCK_DOWNLOAD_WINDOW=%u; clamping\n", MAX_BLOCKS_IN_TRANSIT_PER_PEER, BLOCK_DOWNLOAD_WINDOW);
+        MAX_BLOCKS_IN_TRANSIT_PER_PEER = (int)BLOCK_DOWNLOAD_WINDOW;
+    }
     LogPrintf("Per-peer max blocks in transit: %d\n", MAX_BLOCKS_IN_TRANSIT_PER_PEER);
 
     // Opt-in bulk block streaming (DragonX). Drives the requester branch in SendMessages and, when