add pr-build workflow: build+pack without publishing, upload .b3 artifact

Author: unlucio

.github/workflows/pr-build.yml

@@ -0,0 +1,54 @@
+name: pr-build
+
+# Build, test, pack, and index every plugin in this repo WITHOUT publishing, so a pull request or a
+# branch push gets a downloadable preview of exactly what would ship. Same pipeline as release, driven
+# by the one reusable Bespok3d build Action (b3-builder) with publish: 'false': it still builds the bake
+# payloads, packs the .b3 set, assembles the sub-list index, and runs each plugin's tests, but cuts no
+# GitHub release and registers nothing in the main index. The produced .b3 (plus atoms and the assembled
+# index.json) upload as a run artifact, downloadable from the PR's Checks tab and the Actions run page.
+# Publishing stays main-only, in release.yml.
+on:
+  pull_request:
+  push:
+    branches-ignore: [main]
+    paths:
+      - '*/manifest.json'
+      - '*/files/**'
+      - '*/doc/**'
+      - '*/src/**'
+      - '*/toolchain/**'
+      - '*/tests/**'
+      - .github/workflows/pr-build.yml
+
+# A branch push and the pull_request for the same head would otherwise build twice; collapse them into
+# one run per head branch and cancel a superseded build when a newer commit lands.
+concurrency:
+  group: pr-build-${{ github.event.pull_request.head.ref || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  pr-build:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read   # build only: no release, no index commit, no registration
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: Bespok3d/b3-builder@main
+        with:
+          unit: repo
+          bake: 'true'
+          publish: 'false'
+          atom-repo: ${{ github.repository }}
+          list-name: 'Reference Python Plugins'
+          list-publisher: 'PLACEHOLDER'
+          list-ref-name: 'Reference Python Plugins'
+
+      - uses: actions/upload-artifact@v4
+        with:
+          name: b3-packages
+          path: |
+            dist/*.b3
+            dist/*.atom.json
+            dist/index.json
+          if-no-files-found: error