ci: bump the pinned QuantEcon/mystmd engine to the fork's current main

The HTML build pins an exact SHA so failures stay bisectable, which is the
right call -- but the pin had not moved since 2026-06-12 while the fork's
`main` gained 40 commits, so it was building with `qe-v8` while
build-ipynb.yml, which clones `main`, is on `qe-v10`. The two workflows
were rendering the same lecture commit with materially different engines.

Now pinned to c1f274b8a (2026-09-10, `qe-v10`). The comment no longer
claims the SHA *is* the fork's main, since that is what went stale: it now
records when the SHA was taken and that bumping is a deliberate act.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: mmcky

.github/workflows/ci.yml

@@ -63,17 +63,21 @@ jobs:
           pip install numpyro
           python scripts/test-jax-install.py
 
-      # Pinned to the exact SHA of the fork's main (unmoved since 2026-06-12,
-      # so this is the same engine the 2026-08-03 cold-run failures used).
-      # A branch name here makes the engine a moving target between runs of
-      # the same lecture commit, which makes failures impossible to bisect.
+      # Pinned to an exact SHA rather than a branch: a branch name makes the
+      # engine a moving target between runs of the same lecture commit, which
+      # makes failures impossible to bisect. Bumping it is deliberate.
+      #
+      # This SHA is the fork's main as of 2026-09-10, `qe-v10`. It replaced
+      # 96ee78f5 (2026-06-12, `qe-v8`), which had fallen 40 commits behind, so
+      # the HTML build was running a materially older engine than
+      # build-ipynb.yml -- that workflow clones `main` and so tracks latest.
       - name: Install mystmd (QuantEcon fork)
         run: |
           mkdir -p /tmp/qe-mystmd
           cd /tmp/qe-mystmd
           git init -q
           git remote add origin https://github.com/QuantEcon/mystmd.git
-          git fetch -q --depth 1 origin 96ee78f550ff2e0410730f328352e98d3e29d338
+          git fetch -q --depth 1 origin c1f274b8a27e150474bf2b70bd120627e1bb31ba
           git checkout -q FETCH_HEAD
           git rev-parse HEAD
           bun install