feat: add DCT overflow test patterns for mozilla/mozjpeg#453

Synthetic 8x8 block patterns that trigger 16-bit SIMD forward DCT
overflow when overshoot deringing is enabled. Vertical half-black/
half-white splits produce row-pass intermediates of ±5056; the column
pass sums 8 identical values (40,448 > i16 max 32,767), causing
catastrophic sign flips.

Includes triggering patterns (vertical splits, single block) and
controls (horizontal split, checkerboard) that do NOT trigger.

Author: lilith

imageflow/test_inputs/dct_overflow_patterns/README.md

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+# DCT Overflow Test Patterns
+
+Synthetic images that trigger mozilla/mozjpeg#453: 16-bit SIMD forward DCT overflow
+when overshoot deringing is enabled.
+
+## The Bug
+
+Overshoot deringing pushes level-shifted sample values to ±158 (vs normal ±128).
+The SIMD ISLOW forward DCT uses 16-bit packed arithmetic. After the row pass produces
+intermediate values up to ±5056, the column pass final butterfly sums 8 identical
+row outputs: `8 × 5056 = 40,448`, exceeding the signed 16-bit maximum of 32,767.
+
+The wrapping causes catastrophic sign flips — entire 8×8 blocks have their brightness
+inverted.
+
+**Fix:** Use saturating add/sub (`paddsw`/`psubsw`) instead of wrapping (`paddw`/`psubw`)
+in the final even-part butterfly of the column pass.
+
+## Files
+
+| File | Size | Triggers overflow? | Notes |
+|------|------|--------------------|-------|
+| `left_black_right_white.png` | 64×64 | Yes | Vertical split per 8×8 block |
+| `left_white_right_black.png` | 64×64 | Yes | Inverted vertical split |
+| `single_8x8_half.png` | 8×8 | Yes | Minimal reproducer |
+| `top_black_bottom_white.png` | 64×64 | No | Horizontal split (row pass sees uniform rows) |
+| `checkerboard_8x8.png` | 64×64 | No | Full black/white blocks (no intra-block edge) |
+
+## Why Only Vertical Splits Trigger It
+
+The DCT processes rows first, then columns. A vertical split within an 8×8 block
+means each row sees `[0,0,0,0,255,255,255,255]` (or inverted), producing maximum
+AC energy and intermediate values of ±5056 in the row pass. The column pass then
+sums 8 identical row results.
+
+A horizontal split means each row is either all-black or all-white (DC-only),
+producing zero AC energy. The column pass intermediates stay small.
+
+## Reference
+
+- https://github.com/mozilla/mozjpeg/pull/453
+- Affects: libjpeg-turbo ISLOW FDCT on all SIMD architectures (SSE2, AVX2, NEON, etc.)
+- Quality range: Q1–Q57 (DC quantization value ≥ 14)