Make extensions and futility pruning depend on new seekMate variable
This PR simplifies the recent patch https://github.com/official-stockfish/Stockfish/pull/7040, while still allowing Stockfish to consistently pass the CI `go mate` run involving 61 positions with DTM from 1 to 4 plies.
Results of a deterministic th1 run covering these 61 positions are as follows.
```
Loaded 61 FENs with 61 bm values, with |bm| (min avg max): 1 2 2.
Matetrack started for ./sf.master on matetrack.epd matedtrack.epd with --bmMax 2 --mate 0 ...
100%|███████████████████████████████████████████| 61/61 [00:17<00:00, 3.54it/s]
Using ./sf.master on matetrack.epd matedtrack.epd with --bmMax 2 --mate 0
Engine ID: Stockfish dev-20260819-229f6339
Total FENs: 61
Found mates: 61
Best mates: 61
Best mate statistics:
|bm| = 1 - mates found: 21 = 100.0% of 21; nodes (min avg max): 4 343 1372, depth (min avg max): 1 4 12
|bm| = 2 - mates found: 40 = 100.0% of 40; nodes (min avg max): 16 402441 15454156, depth (min avg max): 1 10 24
All best mates found: 61 = 100.0% of 61; nodes (min avg max): 4 264014 15454156, depth (min avg max): 1 8 24
```
```
Loaded 61 FENs with 61 bm values, with |bm| (min avg max): 1 2 2.
Matetrack started for ./sf.patch on matetrack.epd matedtrack.epd with --bmMax 2 --mate 0 ...
100%|███████████████████████████████████████████| 61/61 [00:02<00:00, 24.64it/s]
Using ./sf.patch on matetrack.epd matedtrack.epd with --bmMax 2 --mate 0
Engine ID: Stockfish dev-20260824-1a3ceee9
Total FENs: 61
Found mates: 61
Best mates: 61
Best mate statistics:
|bm| = 1 - mates found: 21 = 100.0% of 21; nodes (min avg max): 4 343 1372, depth (min avg max): 1 4 12
|bm| = 2 - mates found: 40 = 100.0% of 40; nodes (min avg max): 16 20068 255722, depth (min avg max): 1 9 21
All best mates found: 61 = 100.0% of 61; nodes (min avg max): 4 13277 255722, depth (min avg max): 1 8 21
```
The above shows that patch outperforms master 2s:17s, and 13277:264014 nodes.
In addition, patch shows a healthy increase in (best) mates found on matetrack:
```
Using ./sf.patch on matetrack.epd with --nodes 1000000
Engine ID: Stockfish dev-20260824-1a3ceee9
Total FENs: 6554
Found mates: 3700
Best mates: 2457
```
This compares with 3202, 2263 for master, see https://github.com/vondele/matetrack/blob/c88991a2558b326e2119ca0e5cc9554d60e559ab/matetrack1000000.csv#L4530.
The numbers for the classic mate suite are:
```
Loaded 280 FENs with 280 bm values, with |bm| (min avg max): 10 15 21.
Matetrack started for ./sf.patch on classic280.epd with --nodes 1000000 ...
100%|███████████████████████████████████████████| 56/56 [00:26<00:00, 2.13it/s]
Using ./sf.patch on classic280.epd with --nodes 1000000
Engine ID: Stockfish dev-20260824-1a3ceee9
Total FENs: 280
Found mates: 128
Best mates: 31
```
This compares with 165, 24 for master, see https://github.com/vondele/matetrack/blob/c88991a2558b326e2119ca0e5cc9554d60e559ab/classic1000000.csv#L4530.
Passed STC non-reg:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 41504 W: 10764 L: 10559 D: 20181
Ptnml(0-2): 61, 4620, 11196, 4803, 72
https://tests.stockfishchess.org/tests/view/6a85b5af9960adfadf924292
Passed LTC non-reg:
LLR: 2.94 (-2.94,2.94) <-1.75,0.25>
Total: 111234 W: 28827 L: 28702 D: 53705
Ptnml(0-2): 32, 11680, 32064, 11813, 28
https://tests.stockfishchess.org/tests/view/6a86ef485b1b38ebda864e69
closes https://github.com/official-stockfish/Stockfish/pull/7089
Bench: 2502027