Merge bitcoin/bitcoin#34164: validation: add reusable coins view for ConnectBlock

3e0fd0e4dd refactor: rename will_reuse_cache to reallocate_cache (Andrew Toth)
44b4ee194d validation: reuse same CCoinsViewCache for every ConnectBlock call (Andrew Toth)
8fb6043231 coins: introduce CCoinsViewCache::ResetGuard (Andrew Toth)
041758f5ed coins: use hashBlock setter internally for CCoinsViewCache methods (Andrew Toth)
8dd9200fc9 coins: add Reset on CCoinsViewCache (Andrew Toth)

Pull request description:

  This is the first commit of #31132, which can be merged as an independent change. It has a small benefit on its own, but will help in moving the parent PR forward.

  Add a `Reset()` method to `CCoinsViewCache` that clears `cacheCoins`, `cachedCoinsUsage`, and `hashBlock` without flushing to the `base` view. This allows efficiently reusing a cache instance across multiple blocks.

  Add `CCoinsViewCache::CreateResetGuard` method to return a `CCoinsViewCache::ResetGuard`. The `ResetGuard` automatically calls `Reset()` on destruction. This RAII pattern ensures the cache is always properly reset between blocks.

  Add `m_connect_block_view` as a persistent `CCoinsViewCache` for `ConnectBlock`, avoiding repeated memory allocations.

ACKs for top commit:
  l0rinc:
    ACK 3e0fd0e4dd
  achow101:
    ACK 3e0fd0e4dd
  sedited:
    ACK 3e0fd0e4dd

Tree-SHA512: a95feaa062a9eb7cf7514425a7e7adffd347cd1f7b32b4c1fefcde30002141757c184174702b3104a029dcd33194f8bd734159deebb2e668716089305b42cb00

src/validation.h

@@ -489,6 +489,10 @@ public:
     //! can fit per the dbcache setting.
     std::unique_ptr<CCoinsViewCache> m_cacheview GUARDED_BY(cs_main);
 
+    //! Temporary CCoinsViewCache layered on top of m_cacheview and passed to ConnectBlock().
+    //! Reset between calls and flushed only on success, so invalid blocks don't pollute the underlying cache.
+    std::unique_ptr<CCoinsViewCache> m_connect_block_view GUARDED_BY(cs_main);
+
     //! This constructor initializes CCoinsViewDB and CCoinsViewErrorCatcher instances, but it
     //! *does not* create a CCoinsViewCache instance by default. This is done separately because the
     //! presence of the cache has implications on whether or not we're allowed to flush the cache's