load_model() now defaults to dtype=bf16, fused_decode=True. Both are
free (zero parity regression, bit-exact fused path). Backbone/sigmoid
stay fp32.
Plan updated with benchmark results: tiled+GC = 12x peak memory
reduction at 2048x2048 (27.6GB → 2.3GB), all acceptance criteria met.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>