Merge bitcoin/bitcoin#32694: index: move disk read lookups to base class

029ba1a21d index: remove CBlockIndex access from CustomAppend() (furszy)
91b7ab6c69 refactor: index, simplify CopyHeightIndexToHashIndex to process single block (furszy)
6f1392cc42 indexes, refactor: Remove remaining CBlockIndex* uses in index Rewind methods (Ryan Ofsky)
0a248708dc indexes, refactor: Stop requiring CBlockIndex type to call IsBIP30Unspendable (Ryan Ofsky)
331a25cb16 test: indexes, avoid creating threads when sync runs synchronously (furszy)

Pull request description:

  Combining common refactors from #24230 and #26966, aiming to move both efforts forward while reducing their size and review burden.

  Broadly, #24230 focuses on enabling indexes to run in a separate process, and #26966 aims to parallelize the indexes initial synchronization process. A shared prerequisite for both is ensuring that only the base index class interacts with the node’s chain internals - child index classes should instead operate solely through chain events.

  This PR moves disk read lookups from child index classes to the base index class. It also includes a few documentation improvements and a test-only code cleanup.

ACKs for top commit:
  maflcko:
    review ACK 029ba1a21d 👡
  achow101:
    ACK 029ba1a21d
  TheCharlatan:
    Re-ACK 029ba1a21d
  davidgumberg:
    ACK 029ba1a21d
  mzumsande:
    Code Review ACK 029ba1a21d

Tree-SHA512: f073af407fc86f228cb47a32c7bcf2241551cc89ff32059317eb81d5b86fd5fda35f228d2567e0aedbc9fd6826291f5fee05619db35ba44108421ae04d11e6fb

src/interfaces/chain.h

@@ -327,6 +327,17 @@ public:
         virtual void chainStateFlushed(ChainstateRole role, const CBlockLocator& locator) {}
     };
 
+    //! Options specifying which chain notifications are required.
+    struct NotifyOptions
+    {
+        //! Include undo data with block connected notifications.
+        bool connect_undo_data = false;
+        //! Include block data with block disconnected notifications.
+        bool disconnect_data = false;
+        //! Include undo data with block disconnected notifications.
+        bool disconnect_undo_data = false;
+    };
+
     //! Register handler for notifications.
     virtual std::unique_ptr<Handler> handleNotifications(std::shared_ptr<Notifications> notifications) = 0;