assumevalid: log every script validation state change

The `-assumevalid` option skips script verification for a specified block and all its ancestors during Initial Block Download.
Many new users are surprised when this suddenly slows their node to a halt.
This commit adds a log message to clearly indicate when this optimization ends and full validation begins (and vice versa).

When using `-assumeutxo`, logging is suppressed for the active assumed-valid chainstate and for the background validation chainstate to avoid the confusing toggles.

-------

> cmake -B build && cmake --build build && mkdir -p demo && build/bin/bitcoind -datadir=demo -stopatheight=500 | grep 'signature validation'

```
2025-08-08T20:59:21Z Disabling signature validations at block #1 (00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048).
2025-08-08T20:59:21Z Enabling signature validations at block #100 (000000007bc154e0fa7ea32218a72fe2c1bb9f86cf8c9ebf9a715ed27fdb229a).
2025-08-08T20:59:21Z Disabling signature validations at block #200 (000000008f1a7008320c16b8402b7f11e82951f44ca2663caf6860ab2eeef320).
2025-08-08T20:59:21Z Enabling signature validations at block #300 (0000000062b69e4a2c3312a5782d7798b0711e9ebac065cd5d19f946439f8609).
```

src/validation.cpp

@@ -2580,6 +2580,11 @@ bool Chainstate::ConnectBlock(const CBlock& block, BlockValidationState& state,
              Ticks<SecondsDouble>(m_chainman.time_forks),
              Ticks<MillisecondsDouble>(m_chainman.time_forks) / m_chainman.num_blocks_total);
 
+    if (fScriptChecks != m_prev_script_checks_logged && GetRole() == ChainstateRole::NORMAL) {
+        LogInfo("%s signature validations at block #%d (%s).", fScriptChecks ? "Enabling" : "Disabling", pindex->nHeight, block_hash.ToString());
+        m_prev_script_checks_logged = fScriptChecks;
+    }
+
     CBlockUndo blockundo;
 
     // Precomputed transaction data pointers must not be invalidated

src/validation.h

@@ -550,6 +550,8 @@ protected:
     //! Cached result of LookupBlockIndex(*m_from_snapshot_blockhash)
     mutable const CBlockIndex* m_cached_snapshot_base GUARDED_BY(::cs_main){nullptr};
 
+    std::atomic_bool m_prev_script_checks_logged{true};
+
 public:
     //! Reference to a BlockManager instance which itself is shared across all
     //! Chainstate instances.

test/functional/feature_assumevalid.py

@@ -153,7 +153,7 @@ class AssumeValidTest(BitcoinTestFramework):
         p2p1 = self.nodes[1].add_p2p_connection(BaseNode())
         p2p1.send_header_for_blocks(self.blocks[0:2000])
         p2p1.send_header_for_blocks(self.blocks[2000:])
-
+        with self.nodes[1].assert_debug_log(expected_msgs=['Disabling signature validations at block #1', 'Enabling signature validations at block #103']):
             # Send all blocks to node1. All blocks will be accepted.
             for i in range(2202):
                 p2p1.send_without_ping(msg_block(self.blocks[i]))