Merge bitcoin/bitcoin#26251: refactor: add kernel/cs_main.h

282019cd3d refactor: add kernel/cs_main.* (fanquake)

Pull request description:

  One place to find / include `cs_main`.
  No more:
  > // Actually declared in validation.cpp; can't include because of circular dependency.
  > extern RecursiveMutex cs_main;

  Ultimately, no more need to include `validation.h` (which also includes (heavy/boost filled) `txmempool.h`) everywhere for `cs_main`. See #26087 for another example of why that is useful.

ACKs for top commit:
  ajtowns:
    ACK 282019cd3d

Tree-SHA512: 142835b794873e7a09c3246d6101843ae81ec0c6295e6873130c98a2abfa5f7282748d0f1a37237a779cc71c3bc0a75d03b20313ef5398c83d4814215cbc8287

src/validation.h

@@ -18,6 +18,7 @@
 #include <consensus/amount.h>
 #include <deploymentstatus.h>
 #include <fs.h>
+#include <kernel/cs_main.h> // IWYU pragma: export
 #include <node/blockstorage.h>
 #include <policy/feerate.h>
 #include <policy/packages.h>
@@ -86,7 +87,6 @@ enum class SynchronizationState {
     POST_INIT
 };
 
-extern RecursiveMutex cs_main;
 extern GlobalMutex g_best_block_mutex;
 extern std::condition_variable g_best_block_cv;
 /** Used to notify getblocktemplate RPC of new tips. */