Add bf16 precision option for ViTMatte (--precision, default fp32)

bf16 halves the matting model's activation memory -- its full-resolution
attention is the pipeline's memory peak -- with visually identical alpha
(measured: 0 px alpha deviation > 0.25 on samples; cross-check veto
behaviour unchanged, region IoU 0.984).

Guarded by a hardware check so it never lands on a slow emulation path:
CUDA requires torch.cuda.is_bf16_supported(); CPU requires the same
oneDNN native-bf16 gate PyTorch uses for matmul routing (AVX512-BF16/
AMX). Without support it warns and falls back to fp32 -- on a Zen2 EPYC
the fallback kernels measured 17-370x slower than fp32, so silent bf16
there would be a performance landmine.

Segmentation stays fp32: torchvision deform_conv2d (used by BiRefNet)
has no bf16 CPU kernel, and the segmenter is not the memory peak.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

bgfilter/cli.py

@@ -21,6 +21,7 @@ def main(
     config: Path | None = typer.Option(None, "--config", exists=True, file_okay=True, dir_okay=False),
     model_name: str | None = typer.Option(None, "--model-name"),
     device: str | None = typer.Option(None, "--device"),
+    precision: str | None = typer.Option(None, "--precision", help="ViTMatte precision: fp32 (default) | bf16 (halves matting activation memory; needs bf16-capable hardware, else falls back to fp32)"),
     screen_color: str | None = typer.Option(None, "--screen-color", help="Background colour prior as #RRGGBB (default: auto-detect the flat background colour)"),
     matting_method: str | None = typer.Option(None, "--matting-method"),
     fallback_to_chroma_alpha: bool | None = typer.Option(None, "--fallback-to-chroma-alpha/--no-fallback-to-chroma-alpha"),
@@ -38,6 +39,7 @@ def main(
             load_settings(config),
             model_name=model_name,
             device=device,
+            precision=precision,
             screen_color=screen_color,
             matting_method=matting_method,
             fallback_to_chroma_alpha=fallback_to_chroma_alpha,