Wave 2 scan: 0 matches for composition chains, mode extensions, bg fill. Tasks are harder than expected.

neurogolf_solver/solvers/WAVE2_SCAN.md

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+# Wave 2 Scan Results (2026-04-27)
+
+## Patterns tested against 349 unsolved tasks:
+
+| Pattern | Matches |
+|---------|---------|
+| transform_then_recolor (flip/rot/transpose + color map) | 0 |
+| recolor_then_transform (reverse order) | 0 |
+| row_mode_fill (each row → dominant color) | 0 |
+| col_mode_fill (each col → dominant color) | 0 |
+| fill_bg_with_mode (zeros → global mode) | 0 |
+| fill_bg_with_color (zeros → fixed color) | 0 (PARTIAL on first example only for ~30 tasks) |
+
+## Why bg fill tasks are PARTIAL:
+
+The 53 tasks where "only bg pixels changed" do NOT do simple global fill.
+They fill bg pixels based on **spatial context**:
+- Fill inside a shape boundary
+- Fill adjacent to a specific color
+- Fill based on nearest non-bg neighbor
+
+These require spatial reasoning (flood fill, distance transforms, object detection)
+that cannot be done with simple ReduceSum/ArgMax/Where.
+
+## Conclusion:
+
+Wave 2 analytical solvers have 0 viable targets in this task set.
+The remaining tasks require fundamentally different approaches.