examples: Mandelbrot / Julia escape-time on the ANE

[sbryngelson]

An eye-catching, self-contained demo: the Mandelbrot (and/or a Julia) set by escape-time iteration on the engine, in the family of examples/fluid_vorticity.py / examples/reaction_diffusion.py.

Approach:

• Build a coordinate grid of c = cr + i*ci over the complex plane. Complex numbers are carried as two real tensors zr, zi (the ANE is real-valued; aneforge/fft.py already carries spectra this way).
• One iteration is elementwise: zr, zi = zr*zr - zi*zi + cr, 2*zr*zi + ci, then an escape mask where zr*zr + zi*zi > 4. Accumulate the escape iteration count with select / maximum (in graph.py). Each iteration is one on-engine forward pass over the whole grid.
• After the fixed iteration budget, map the smooth iteration count through a colormap and save a PNG (PIL, like the sibling demos). Optionally sweep a Julia c to make a short animation.

Deliverable: examples/mandelbrot.py (one file, docstring), plus an examples/README.md entry.

Testing: spot-check a handful of grid points against a plain numpy escape-time reference. Requires any Apple Silicon Mac. Follow reaction_diffusion.py for the render, _common.py for setup.