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).
```

…hange

fab2980 assumevalid: log every script validation state change (Lőrinc)

Pull request description:

  The `-assumevalid` option skips script verification for a specified block and all its ancestors during Initial Block Download.
  Many new [users are surprised](bitcoin#32832) 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).

  <details>
  <summary>Testing instructions</summary>

  The behavior can easily be tested by adding this before the new log:
  ```C++
      // TODO hack to enable/disable script checks based on block height for testing purposes
           if (pindex->nHeight < 100) fScriptChecks = false;
      else if (pindex->nHeight < 200) fScriptChecks = true;
      else if (pindex->nHeight < 300) fScriptChecks = false;
      else if (pindex->nHeight < 400) fScriptChecks = true;
  ```
  and exercise the new code with:
  ```bash
  cmake -B build && cmake --build build && mkdir -p demo && build/bin/bitcoind -datadir=demo -stopatheight=500 | grep 'signature validation'
  ```
  showing something like:
  * Disabling signature validations at block #1 (00000000839a8e6886ab5951d76f411475428afc90947ee320161bbf18eb6048).
  * Enabling signature validations at block #100 (000000007bc154e0fa7ea32218a72fe2c1bb9f86cf8c9ebf9a715ed27fdb229a).
  * Disabling signature validations at block #200 (000000008f1a7008320c16b8402b7f11e82951f44ca2663caf6860ab2eeef320).
  * Enabling signature validations at block #300 (0000000062b69e4a2c3312a5782d7798b0711e9ebac065cd5d19f946439f8609).

  </details>

ACKs for top commit:
  achow101:
    ACK fab2980
  ajtowns:
    crACK fab2980
  davidgumberg:
    untested crACK bitcoin@fab2980

Tree-SHA512: e90b66f7423b639356daace476942ce83e65e70466544394cbe2f15738bdbf716163eaf590c64c5448f9b41aeeaafe3342c48c6a7a478678a70b0310ca94e11d