Fix index out of bounds panic in iterative_parallel_mvv_lva when depth >= 10

[Copilot]

IterativeSearcher::best_move panics when called with depth >= 10 because the iterative deepening loop indexes into PLYS_SEQ (length 10) with the raw depth value, which goes out of bounds.

index out of bounds: the len is 10 but the index is 10

• Clamp the PLYS_SEQ index to the last valid entry so depths beyond the table size use the plateau value (3)
• Add regression test with the exact FEN from the bug report

// Before
let returned_b_move = jamboree(&mut b, alpha, beta, i, PLYS_SEQ[i as usize]);

// After
let plys_idx = std::cmp::min(i as usize, PLYS_SEQ.len() - 1);
let returned_b_move = jamboree(&mut b, alpha, beta, i, PLYS_SEQ[plys_idx]);

Original prompt

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This section details on the original issue you should resolve

<issue_title>iterative_parallel_mvv_lva panic</issue_title>
<issue_description>```rust
use pleco::{bots::IterativeSearcher, tools::Searcher, Board};

fn main() {
let mut clipboard = arboard::Clipboard::new().unwrap();
let mut last_clipboard_text = "".to_string();
loop {
let current_clipboard_text = clipboard.get_text().unwrap();
if current_clipboard_text.starts_with("FEN: ") {
if current_clipboard_text != last_clipboard_text {
let fen = current_clipboard_text[5..].trim();
let board = Board::from_fen(&fen).unwrap();
let depth = 10;
println!("Searching depth {depth} for FEN: {fen}");
let best_move = IterativeSearcher::best_move(board, depth);
println!("FEN: {fen} Best move: {}", best_move);
last_clipboard_text = current_clipboard_text.clone();
}
}
std::thread::sleep(std::time::Duration::from_millis(100));
}
}

$ cargo run
Compiling chess_bot v0.1.0 (/Users/brandon/Desktop/chess_bot)
Finished dev profile [unoptimized + debuginfo] target(s) in 0.21s
Running /Users/brandon/.cargo/target/debug/chess_bot
Searching depth 10 for FEN: 1k6/R7/8/7B/7P/8/5Q2/B3K2R w K - 11 47
thread 'main' panicked at /Users/brandon/.cargo/registry/src/index.crates.io-6f17d22bba15001f/pleco-0.5.0/src/bots/iterative_parallel_mvv_lva.rs:39:64:
index out of bounds: the len is 10 but the index is 10
note: run with RUST_BACKTRACE=1 environment variable to display a backtrace

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes iterative_parallel_mvv_lva panic #162

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