opusenc: memoise the pitch contour correlations

The contour search tries 5 base lags times 34 codebook contours times 4
subframes, 680 normalised correlations, but only about 4 x 30 distinct
(subframe, lag) pairs occur. Cache them per call. The values are the same
f64 results as before, so the bitstream does not change (cmp on 24 SILK and
hybrid configurations of 90 s, mono and stereo, speech and music).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/opusenc/src/silk/analysis.rs

@@ -229,9 +229,20 @@ pub fn pitch_search(x: &[f32], start: usize, len: usize, fs_khz: usize) -> Pitch
     let score = full[(lag - min_lag) as usize];
     // Contour: per-subframe lag offsets around the chosen lag.
     let cb: &[[i32; 4]] = if fs_khz == 8 { &CB_NB_20 } else { &CB_WB_20 };
+    // The contour search revisits the same (subframe, lag) pairs for neighbouring base lags and
+    // codebook entries, so each correlation is computed once.
+    let span = (18 * fs - min_lag + 1) as usize;
+    let memo = std::cell::RefCell::new(vec![f64::NAN; 4 * span]);
     let sub_score = |k: usize, l: i32| -> f64 {
         let l = l.clamp(min_lag, 18 * fs);
-        ncorr(x, start + k * sub..start + (k + 1) * sub, l as usize)
+        let slot = k * span + (l - min_lag) as usize;
+        let cached = memo.borrow()[slot];
+        if !cached.is_nan() {
+            return cached;
+        }
+        let v = ncorr(x, start + k * sub..start + (k + 1) * sub, l as usize);
+        memo.borrow_mut()[slot] = v;
+        v
     };
     let mut best = (f64::MIN, lag, 0usize);
     for l0 in (lag - 2).max(min_lag)..=(lag + 2).min(max_lag) {