Add executable demo notebook: ppstats vs scipy.stats with interactive explorers

[reedpen]

Summary

Adds demo/ppstats_demo.ipynb, an executable tour of the full capability story:

• Kernel source inspection — inspect.getsource on real @guvectorize/@vectorize kernels: the source is plain typed Python.
• Side-by-side accuracy vs scipy.stats — descriptive reductions and distribution kernels checked with the documented tolerance models: 1e-12 relative for direct-formula kernels on a fixed grid, a purely absolute 2e-7 check for norm_cdf (ppspecial's ndtr bound is absolute), and the bench harness's 1e-9 allowance for random-data cancellation in the reductions (explicitly labeled as such).
• Two execution modes, one source — interpreted family modules vs the compiled ppstats_native extension, asserted to agree at 1e-13.
• gufunc semantics — (batch, n) reductions and loc/scale broadcasting contrasted with scipy's axis=/broadcast calls.
• Interactive explorers (ipywidgets) — a distribution explorer (family dropdown + loc/scale sliders, live ppstats/scipy overlay with abs/rel diff and timing readouts) and a performance explorer (size selector 1e3–1e6 + function dropdown, live speedup bars).
• Static performance summary at N=1e6, honestly including the known loss cases (mean on long vectors, norm_ppf), with perf prose worded as commonly-observed-on-linux-64 rather than absolute claims.

Environment

Ships as a pixi demo feature in its own solve group (scipy, matplotlib, ipywidgets, jupyterlab, nbconvert; python pinned 3.14 to match the compiled extension ABI) — scipy stays out of [project].dependencies and the dev feature, per the working rules. Default/dev lock entries are byte-identical to main. New tasks: demo-build, demo-exec (headless execution check), demo.

pixi run -e demo demo-build
pixi run -e demo demo

Verification

• pixi install -e demo solves; default/dev solves unchanged
• pixi run -e demo demo-build → 27 ufuncs registered
• pixi run -e demo demo-exec → exit 0, zero error outputs (norm_cdf max abs error 9.6e-8 vs 2e-7 allowance)
• pixi run -e dev test → 198 passed, unchanged profile
• Committed notebook has cleared outputs; executed copy and built .so are gitignored