fix: make nCoinCacheUsage std::atomic to close an adaptive-dbcache data race

The scheduled AdjustCoinCacheForMemoryPressure task writes nCoinCacheUsage from
the scheduler thread holding no lock, while cs_main-holding threads
(FlushStateToDisk, VerifyDB) read it -- an unsynchronized read/write of a
non-atomic size_t (C++ UB). Make it std::atomic<size_t>; correct the comment
that incorrectly described the access as lock-free/race-free.

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

src/main.h

@@ -43,6 +43,7 @@
 #include "txmempool.h"
 #include "uint256.h"
 
+#include <atomic>
 #include <algorithm>
 #include <exception>
 #include <map>
@@ -189,7 +190,7 @@ extern bool fCheckpointsEnabled;
 // TODO: remove this flag by structuring our code such that
 // it is unneeded for testing
 extern bool fCoinbaseEnforcedProtectionEnabled;
-extern size_t nCoinCacheUsage;
+extern std::atomic<size_t> nCoinCacheUsage;
 extern CFeeRate minRelayTxFee;
 extern int64_t nMaxTipAge;