feat: Use locked Pixi environment for Binder deployment (#110)

* Use binder/postBuild pattern to install conda explicit spec export
  of Pixi environment into Binder conda environment.
* Remove binder/apt.txt and binder/requirements.txt.
* Update book/introduction.md to note that for pip or uv users the
  dependencies are in book/requirements.txt.

Author: matthewfeickert

binder/apt.txt

@@ -1,11 +1,16 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
-set -e
+set -ex
 
-# Make ROOT available for comparisons and workspace use
-# c.f. https://github.com/conda-forge/root-feedstock
-conda config --add channels conda-forge
-conda config --set channel_priority strict
+# Install Pixi
+curl -fsSL https://pixi.sh/install.sh | sh
+echo 'eval "$(pixi completion --shell bash)"' >> ~/.bashrc
+. ~/.bashrc
+# Sourcing ~/.bashrc doesn't seem to pick up changes, so manually set PATH
+export PATH="~/.pixi/bin:${PATH}"
 
-# Use mamba to speed up install
-mamba install --yes root_base
+# Export workspace to conda explicit spec
+pixi workspace export conda-explicit-spec --platform linux-64 binder/
+
+# Install environment
+mamba install --name notebook --file binder/default_linux-64_conda_spec.txt

binder/postBuild

@@ -1 +1 @@
-python-3.12
+python-3.13

binder/requirements.txt

@@ -178,13 +178,7 @@ pixi run start
 
 #### Using `pip`
 
-then install from the included `requirements.txt` in the top level `binder/` directory of [the source repository](https://github.com/pyhf/pyhf-tutorial)
-
-```
-(pyhf-tutorial) $ python -m pip install --upgrade --requirement binder/requirements.txt
-```
-
-If you want to also get the dependencies to build and explore the Jupyter Book form of the tutorial you can install them with
+then install from the included `requirements.txt` in the top level `book/` directory of [the source repository](https://github.com/pyhf/pyhf-tutorial)
 
 ```
 (pyhf-tutorial) $ python -m pip install --upgrade --requirement book/requirements.txt

binder/runtime.txt

@@ -1 +1,12 @@
-jupyter-book==0.15.1
+pyhf[xmlio,minuit,contrib]==0.7.6
+# visualization
+ipywidgets~=8.1.8
+ipympl~=0.9.8
+pandas~=2.3.3
+altair~=6.0.0
+rich==14.2.0
+# jupyter notebooks
+notebook>=7.0.0
+jupyterlab>=4.0.3
+# book
+jupyter-book==2.0.2

book/introduction.md

@@ -21,6 +21,7 @@ altair = ">=6.0.0,<7"
 rich = ">=14.2.0,<15"
 # comparison
 root_base = ">=6.36.4,<7"
+jq = ">=1.8.1,<2"
 
 [feature.lab.dependencies]
 notebook = ">=7.5.0,<8"