Smoothen seekMate trigger
This PR replaces the binary `seekMate` trigger with a smoother version.
The motivation for the idea comes from #7179. There it was found that master for some of the 4-ply mate puzzles reached a root eval just below the old trigger of 2000, and stayed at that for many iterations. In multi-threaded analysis this could at times lead to a failure of our matetrack CI.
This patch replaces the old trigger of 2000 (from depth 16 onwards) with a depth-dependent value. The smooth curve `750 + 220000 / (d*d)` approaches 750 towards infinity and goes through the points (10, 2950), (16, 1609), (20, 1300), (50, 838). These raw eval values for the current WDL model and at the material value 58 correspond to 970cp, 529cp, 427cp, 276cp, and all have W=100%.
Indeed, patch on average and in the worst case finds the mates from our 4-ply CI set faster than master:
```
Using ./sf.master on mates2head.epd with --mate 0 --threads 1
Engine ID: Stockfish dev-20260922-0a215d6c
Total FENs: 1000
Found mates: 1000
Best mates: 1000
Best mate statistics:
|bm| = 1 - mates found: 4 = 100.0% of 4; nodes (min avg max): 4 69 220, depth (min avg max): 1 1 1
|bm| = 2 - mates found: 996 = 100.0% of 996; nodes (min avg max): 4 127441 53095641, depth (min avg max): 1 9 78
All best mates found: 1000 = 100.0% of 1000; nodes (min avg max): 4 126931 53095641, depth (min avg max): 1 9 78
```
```
Using ./sf.patch on mates2head.epd with --mate 0 --threads 1
Engine ID: Stockfish dev-20260926-m-ff6998f6
Total FENs: 1000
Found mates: 1000
Best mates: 1000
Best mate statistics:
|bm| = 1 - mates found: 4 = 100.0% of 4; nodes (min avg max): 4 69 220, depth (min avg max): 1 1 1
|bm| = 2 - mates found: 996 = 100.0% of 996; nodes (min avg max): 4 23482 8342063, depth (min avg max): 1 8 63
All best mates found: 1000 = 100.0% of 1000; nodes (min avg max): 4 23389 8342063, depth (min avg max): 1 8 63
```
So patch finds the mates on average with 23K nodes, master with 127K. And the maximum nodes are about 8M vs 53M in favour of patch.
The patch passed STC and LTC non-reg vs. master with `seekMate` disabled.
STC:
LLR: 2.93 (-2.94,2.94) <-1.75,0.25>
Total: 173504 W: 44066 L: 44003 D: 85435
Ptnml(0-2): 215, 18265, 49709, 18368, 195
https://tests.stockfishchess.org/tests/view/6ab230bbe28108115db44437
LTC:
LLR: 2.96 (-2.94,2.94) <-1.75,0.25>
Total: 375264 W: 96398 L: 96530 D: 182336
Ptnml(0-2): 80, 36928, 113741, 36810, 73
https://tests.stockfishchess.org/tests/view/6ab56fae83e20a0615e67adb
And even though that was not the original intention of the patch, it also shows a nice bump in our matetrack stats:
```
Using ./sf.patch on matetrack.epd with --nodes 1000000
Engine ID: Stockfish dev-20260926-m-ff6998f6
Total FENs: 6554
Found mates: 3765
Best mates: 2535
```
This compares with 3595, 2382 for master, see [matetrack](https://github.com/vondele/matetrack/blob/edee34f4ba4c45c2f7141ec0e7503baf3a1f7ef1/matetrack1000000.csv#L4588).
For the suite of classic mate puzzles the difference is minor: 107, 20 vs 107, 12 in favour of patch.
Fixes #7179.
closes https://github.com/official-stockfish/Stockfish/pull/7201
Bench: 1514301