0.9: Affine analytic cost model, pad-to-tile scheduling, better tiling for convolutions

From the README:

• 0.9: Program search and optimization.
  • Native affine program analysis and a Schedule.op_legality oracle; an analytic roofline cost model that picks untuned defaults and pre-filters the autotune beam (advisory throughout).
  • Deterministic split reductions, pad-to-tile scheduling (PADTO), static partitioning, epilogue fusion, and launch-time symbolic extents.
  • Convolution schedule families: implicit GEMM via packing Stage, blocked tile flavors, epilogue twins, compacting strided-row staging, and clamped-window pooling.
  • Mixed precision: a precision-assignment policy, master weights with cast twins, dynamic loss scaling with a fused on-device gate, tf32 matmuls, and forward-only reduced precision by load-time conversion.
  • Liveness-based buffer aliasing; a packed uniform that is total over shapes and now backs default parameter initialization.
  • Search survivability: typed candidate-failure containment across Metal/CUDA/HIP, HIP scratch pre-validation, no unparallelized GPU dispatches, and a decline census that accounts for every refusal.
  • A cross-machine benchmark sweep on Metal, CUDA and HIP, with the reports checked in under benchmarks/.