Build CI the way GitHub Pages does, so CI predicts the deploy

.github/Gemfile.pages

@@ -0,0 +1,15 @@
+# The Gemfile GitHub Pages actually builds with — not the one in the repo root.
+#
+# `pages-build-deployment` ignores our Gemfile and runs the `github-pages` gem
+# (currently 232, which pins Jekyll 3.10). Jekyll 3.10 forgives less Liquid than
+# the Jekyll 4 we develop against: `where_exp` takes exactly one comparison and
+# then demands end-of-string, so `and` / `or` inside the expression — a Jekyll 4
+# feature — parses locally and dies on deploy. roadmap.md already carries a
+# comment about that trap; this file is what makes CI catch the next one.
+#
+# CI builds against this file so it predicts the deploy. Do not pin a version
+# here: tracking whatever `github-pages` currently resolves to is the point.
+
+source "https://rubygems.org"
+
+gem "github-pages", group: :jekyll_plugins

.github/workflows/ci.yml

@@ -3,8 +3,11 @@ on:
   pull_request:
   push:
     branches: [main]
+
 jobs:
+  # The Jekyll we develop against. Fast, and matches a local `jekyll serve`.
   build:
+    name: Jekyll 4 (what we develop on)
     runs-on: ubuntu-latest
     steps:
       - uses: actions/checkout@v4
@@ -13,3 +16,20 @@ jobs:
           ruby-version: '3.2'
           bundler-cache: true
       - run: bundle exec jekyll build
+
+  # The build that actually predicts the deploy. GitHub Pages ignores the root
+  # Gemfile and runs the `github-pages` gem (Jekyll 3.10), which accepts less
+  # Liquid than Jekyll 4 — see .github/Gemfile.pages. Without this job, CI stays
+  # green while pages-build-deployment fails and the site silently serves stale.
+  pages-parity:
+    name: GitHub Pages (what actually deploys)
+    runs-on: ubuntu-latest
+    env:
+      BUNDLE_GEMFILE: ${{ github.workspace }}/.github/Gemfile.pages
+    steps:
+      - uses: actions/checkout@v4
+      - uses: ruby/setup-ruby@v1
+        with:
+          ruby-version: '3.2'
+          bundler-cache: true
+      - run: bundle exec jekyll build

.gitignore

@@ -4,4 +4,5 @@ _site/
 vendor/
 .bundle/
 Gemfile.lock
+Gemfile.pages.lock
 .DS_Store