Avoid recomputing threat/pp accumulation for some king moves.
Since the separate threat and psq accumulators were merged in 7c7fe322eae6fa3a594c036eb4fc2553fd4e6a11, any king move requires a full recomputation of the threat (and now also pawn-pair) accumulation, even though these only change when the king switches between the left and right half of the board. This patch adds a method `update_accumulator_hybrid` which incrementally computes the accumulator change of a king move as
```
<new acc> = <previous acc> - <previous psq acc> + <new psq acc> + <threat/pp changes>
```
thus avoiding re-adding all threat/pp features. `<previous psq acc>` and `<new psq acc>` can be computing efficiently using the accumulator cache ("Finny table"). When there are few pieces on the board there typically are only few threat/pp features so computing their sum from scratch is cheaper than computing `<previous psq acc>` from the cache entry. Therefore we only do "hybrid" updates for positions with at least `MIN_PC_COUNT_HYBRID = 15` pieces. For simplicity this also doesn't handle castling moves.
passed STC: https://tests.stockfishchess.org/tests/view/6a5b8a7d5529b8472df8158b
LLR: 2.94 (-2.94,2.94) <0.00,2.00>
Total: 37792 W: 9855 L: 9557 D: 18380
Ptnml(0-2): 65, 3955, 10578, 4213, 85
local benchmark:
```
Result of 100 runs (counting nodes/1B cycles)
=============================================
base (...tockfish.f4bcd4) = 264725 +/- 3700
test (...tockfish.4e77c9) = 266318 +/- 3701
speedup % = +0.60 +/- 0.08
[95% CI; t-test]
likelihood(speedup > 0) = 1.0000
CPU: 16 x Intel(R) Core(TM) Ultra 9 185H
Hyperthreading: on
```
I don't know if `MIN_PC_COUNT_HYBRID = 15` is optimal. On my computer (avx-vnni) 12 was the optimum, but that did not perform well [on fishtest](https://tests.stockfishchess.org/tests/view/6a5769345529b8472df80d7e). It would be good if other devs could try benchmarking different thresholds and report.
The first version of this patch was coded up by gpt-5.5-high. I made many changes, but probably most of the lines of code are LLM-written.
closes https://github.com/official-stockfish/Stockfish/pull/6990
No functional change