Sync speed + anti-fork hardening (v5.8.5)

- In-memory UTXO cache (2M entries, read-through with negative caching)
- Signature cache upgrade (unordered_set, 200K entries, 64-bit compact keys)
- Speed-weighted peer block assignment (fast peers get more blocks)
- 500-block max reorg depth (finality limit post-IBD)
- 7-day coin age soft cap (prevents stake surprise attacks)
- Timestamp tiebreaker for equal-trust fork resolution
- 30s stake cooldown after orphaned block (reduces fork oscillation)
- Slow-peer eviction (disconnect 0-block peers after 3min during sync)
- Only push new blocks to near-tip peers (within 10 blocks)
- Smart orphan eviction (FIFO oldest-first instead of random)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/miner.cpp

@@ -446,9 +446,20 @@ void StakeMiner(CWallet *pwallet)
         {
             printf("StakeMiner(): A proof-of-stake block has been found! %s\n", pblock->GetHash().ToString().c_str());
             SetThreadPriority(THREAD_PRIORITY_NORMAL);
-            CheckStake(pblock.get(), *pwallet);
+            bool fAccepted = CheckStake(pblock.get(), *pwallet);
             SetThreadPriority(THREAD_PRIORITY_LOWEST);
-            MilliSleep(500);
+            if (fAccepted)
+            {
+                MilliSleep(500);
+            }
+            else
+            {
+                // Block was orphaned or rejected — apply a cooldown to reduce
+                // fork oscillation. Without this, the staker immediately retries
+                // with a different timestamp, potentially creating competing forks.
+                printf("StakeMiner(): block not accepted, cooldown 30s\n");
+                MilliSleep(30000);
+            }
         }
         else
           MilliSleep(500);