tools: add diffedit3 as a dotslash tool

.github/actions/install-dotslash/action.yml

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+name: Install DotSlash
+description: Install DotSlash on the current system
+inputs:
+  version:
+    description: "version of dotslash"
+    required: false
+    default: "0.4.1"
+runs:
+  using: "composite"
+  steps:
+    - name: Install DotSlash
+      shell: bash
+      run: |
+        mkdir -p $RUNNER_TEMP/dotslash
+        echo "$RUNNER_TEMP/dotslash" >> $GITHUB_PATH
+
+        if [[ "$RUNNER_OS" == "Windows" ]]; then
+          C:\\msys64\\usr\\bin\\wget.exe https://github.com/facebook/dotslash/releases/download/v0.4.1/dotslash-windows.v0.4.1.tar.gz
+          tar xf dotslash-windows.v0.4.1.tar.gz
+          mv dotslash.exe $RUNNER_TEMP/dotslash
+        else
+          if [[ "$RUNNER_OS" == "macOS" ]]; then
+            wget https://github.com/facebook/dotslash/releases/download/v0.4.1/dotslash-macos.v0.4.1.tar.gz
+            tar xf dotslash-macos.v0.4.1.tar.gz
+          elif [[ "$RUNNER_OS" == "Linux" ]]; then
+            wget https://github.com/facebook/dotslash/releases/download/v0.4.1/dotslash-ubuntu-22.04.x86_64.v0.4.1.tar.gz
+            tar xf dotslash-ubuntu-22.04.x86_64.v0.4.1.tar.gz
+          fi
+          mv dotslash $RUNNER_TEMP/dotslash
+        fi

.github/workflows/check-dotslash.yml

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+name: check dotslash files
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+permissions: read-all
+
+jobs:
+  nix:
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - os: ubuntu-latest
+            abi: x86_64-unknown-linux-gnu
+          - os: windows-latest
+            abi: x86_64-pc-windows-msvc
+          - os: macos-latest
+            abi: aarch64-apple-darwin
+    runs-on: ${{ matrix.os }}
+    timeout-minutes: 15 # NOTE (aseipp): keep in-sync with the build.yml timeout limit
+
+    name: check dotslash files
+    steps:
+      - uses: actions/checkout@692973e3d937129bcbf40652eb9f2f61becf3332
+      - name: Install DotSlash
+        uses: ./.github/actions/install-dotslash
+      - shell: bash
+        run: |
+          for x in $(ls tools/bin); do
+            if [ "$x" = "README.md" ]; then
+              continue;
+            fi
+            if ! dotslash "tools/bin/$x" --help; then
+              echo "error: $x failed to run on ${{ matrix.abi }}"
+              exit 1
+            fi
+          done

tools/bin/README.md

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+# `dotslash` executables
+
+This directory contains various [DotSlash](https://github.com/facebook/dotslash)
+executables: portable executables that are downloaded on demand.
+
+DotSlash lets us have our tools available on any platform without the need to
+install them, in a version-controllable and repeatable way that doesn't bloat
+repositories. This helps make sure developers can have consistent environments,
+if they wish to opt in.
+
+- [Install DotSlash](https://dotslash-cli.com/docs/installation/)
+  - TL;DR cargo users: `cargo install dotslash`
+  - TL;DR nix users: `nix profile install nixpkgs#dotslash`
+  - TL;DR everyone else: [Download the latest release](https://github.com/facebook/dotslash/releases)
+
+> [!TIP]
+> DotSlash files are most useful for cross-platform tools we want to provide
+> _developers_ on _all_ platforms — including Windows! Some other tools
+> may also be provided by e.g. Nix or Cargo.
+
+Once `dotslash` is somewhere in your `$PATH`, add these files to your `$PATH`
+too:
+
+```bash
+export PATH=$(jj root)/tools/bin:$PATH
+```
+
+Then tools like `diffedit3` will work with a small one-time startup penalty to
+download the executable.
+
+If you're curious, just open any of the DotSlash files in this directory in
+your EDITOR; they are merely simple JSON files.
+
+## Windows users
+
+Windows users need to invoke the path to the dotslash file with the `dotslash`
+command:
+
+```
+dotslash .\tools\bin\diffedit3
+```
+
+This is a technical limitation that will be alleivated in the future (once the
+`windows_shim` for DotSlash is released.)

tools/bin/diffedit3

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+#!/usr/bin/env dotslash
+
+{
+  "name": "diffedit3",
+  "platforms": {
+    "macos-aarch64": {
+      "size": 2285223,
+      "hash": "blake3",
+      "digest": "2b4fdb0587aece5658030b68dc7271973b5c731e0c2493efd3aa87ff3f445a61",
+      "format": "tar.gz",
+      "path": "diffedit3",
+      "providers": [
+        {
+          "url": "https://github.com/ilyagr/diffedit3/releases/download/v0.4.0/diffedit3-v0.4.0-aarch64-apple-darwin.tar.gz",
+        }
+      ]
+    },
+    "windows-x86_64": {
+      "size": 1900470,
+      "hash": "blake3",
+      "digest": "462c4fc77655e0688e5317fcdc2a4a6af326b7f04912fc8c7489028b56e50a59",
+      "format": "zip",
+      "path": "diffedit3.exe",
+      "providers": [
+        {
+          "url": "https://github.com/ilyagr/diffedit3/releases/download/v0.4.0/diffedit3-v0.4.0-x86_64-pc-windows-msvc.zip"
+        }
+      ]
+    },
+    "linux-x86_64": {
+      "size": 3498501,
+      "hash": "blake3",
+      "digest": "84557e1409e0e4c76d80ff0758ab5995ec40dd7a151bfdef7858c54d71d43d9a",
+      "format": "tar.gz",
+      "path": "diffedit3",
+      "providers": [
+        {
+          "url": "https://github.com/ilyagr/diffedit3/releases/download/v0.4.0/diffedit3-v0.4.0-x86_64-unknown-linux-musl.tar.gz"
+        }
+      ]
+    }
+  }
+}