[Repo Assist] Perf: replace multi-pass Seq.pearsonWeighted with 2-pass single-allocation impl

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Problem

Seq.pearsonWeighted traversed the three input sequences ~12+ times:

3×  Seq.length (length guard, one full pass each)
+
weightedCorrelation called weightedCoVariance three times (cov_xy, var_x, var_y)
  each weightedCoVariance called weightedMean twice (2 passes) + 1 pass for the sum
  → 3 × 3 = 9 passes
Total ≈ 12 passes over the input data

For n elements this means roughly 12n floating-point reads. Internal allocation was also high: three Seq-based closures (fold2, map3, sum) were created per call.

Fix

Two-pass, single-allocation implementation:

1. Convert inputs to arrays once — 3 passes, avoids repeated enumeration
2. Pass 1: compute weighted means x̄_w and ȳ_w in a single for loop
3. Pass 2: compute weighted covariance and both weighted variances simultaneously from the means — one for loop

// Before (conceptually, ~12 passes)
weightedCorrelation seq1 seq2 weights   // calls weightedCoVariance 3×, each calls weightedMean 2×

// After (5 passes: 3 toArray + 1 means + 1 cov/var)
let xMean, yMean = ...  // single pass
let covXY, varX, varY = ...  // single pass
covXY / sqrt(varX * varY)   // no extra pass

The wSum denominator cancels in cov / sqrt(varX × varY), so the formula is mathematically identical to the original.

Performance Impact

n	Before (est. passes)	After (passes)	Improvement
1,000	~12,000 reads	~5,000 reads	~2.4×
100,000	~1,200,000 reads	~500,000 reads	~2.4×
1,000,000	~12,000,000 reads	~5,000,000 reads	~2.4×

In practice the speedup is higher because the old code also performed repeated heap allocations for the Seq.map3/Seq.fold2 closures.

Test Status

5 new regression tests added:

• docstring example — matches the -0.9764158959 value from the XML doc
• ofTriples matches pearsonWeighted — verifies API consistency
• uniform weights: perfect positive correlation — r = 1.0
• uniform weights: perfect negative correlation — r = −1.0
• mismatched length throws — error handling

✅ Build: succeeded
✅ Tests: 1196/1196 passed (1191 existing + 5 new)

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