Fix SIMD scatter kernel rank computation bug

[konard]

Summary

This PR fixes the GPU radix sort (SIMD) verification failure reported in Issue #17.

Root Cause

The bug was in shaders/radix_sort.metal at lines 325-328 in the radix_scatter_simd kernel:

// BEFORE (BUG):
if (simd_lane == 0) {
    simd_digit_counts[simd_group_id * RADIX_SIZE + digit] = simd_count;
}

The Problem: Only simd_lane == 0 wrote to simd_digit_counts, but it only wrote the count for its own digit. This left counts for all other digits in the SIMD group as zero.

Example: If SIMD group 0 has threads with digits [3, 5, 3, 7, 5, ...]:

• Lane 0 (digit=3) writes count for digit 3
• Digits 5, 7 get count=0 written (from initialization)
• Threads with digits 5, 7 later read zeros, compute wrong ranks, write to wrong positions

The Fix

Changed the condition from simd_lane == 0 to simd_rank == 0:

// AFTER (FIXED):
if (simd_rank == 0) {
    simd_digit_counts[simd_group_id * RADIX_SIZE + digit] = simd_count;
}

This ensures that the first thread for each unique digit in the SIMD group writes its count, so all digits present get their counts correctly recorded.

Changes

File	Description
shaders/radix_sort.metal	Fixed condition from simd_lane == 0 to simd_rank == 0
docs/case-studies/issue-17/analysis.md	Added comprehensive case study with root cause analysis

Test Plan

• Local cargo fmt -- --check passes
• Local cargo clippy -- -D warnings passes
• Local cargo test passes (47 tests)
• CI build passes on Linux
• macOS test: cargo run --release -- 2684354 (requires manual verification on Apple Silicon)

Expected Results After Merge

Running cargo run --release -- 2684354 on macOS with Apple Silicon should show:

• GPU radix sort (SIMD) passes verification
• Results match CPU sort
• SIMD version remains ~1.8x faster than basic GPU radix sort

Fixes #17

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[konard]

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