CuTile Autotuning

Add autotuning to CuTile kernels using the exhaustive_search API with tune-once/cache/direct-launch pattern.

Instructions

Follow the decision tree to classify the kernel, design a search space, implement the tune-once/cache/launch pattern, and validate performance.

1. Classify — use the Decision Tree to determine search dimensions (occupancy-only vs full tile search)
2. Design search space — select the matching template from references/kernel-type-templates.md; prune to ≤ 30 configs in the final code via arch filters (directed exploration probes may temporarily exceed this — see Design Philosophy)
3. Implement — add exhaustive_search + cache + ct.launch following the Step-by-Step Workflow; handle in-place writes with split-buffer if needed
4. Test — run correctness with autotune enabled and with DISABLE_AUTOTUNE=1
5. Validate — A/B benchmark against fixed best-known config; see references/search-strategies.md
6. Shrink — prune dead-weight configs that never win, targeting ≤ 8 configs per architecture to minimize compilation cost (Step 10)

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