fix(website): pin UV_PYTHON=3.13 for the Amplify build

The build phase died in resolve_sources.py with `ModuleNotFoundError: No module
named 'tomllib'`. tomllib is stdlib only in Python 3.11+, but the repo declares
requires-python ">=3.10" with no version pin, so uv on Amplify's Amazon Linux
created the venv with a <=3.10 interpreter. (Locally and on GH Actions uv happens
to pick 3.13.)

Set UV_PYTHON=3.13 so uv fetches/uses 3.13 for both the root env and the
notebook emitter project (both declare >=3.10 and resolve to 3.13 locally),
giving resolve_sources.py + gen_compat_matrix.py their tomllib.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Roger-luo

amplify.yml

@@ -23,6 +23,13 @@ env:
     NODE_VERSION: "24"
     PNPM_VERSION: "11"
     UV_VERSION: "0.5.1"
+    # Pin the Python uv uses. The repo declares requires-python ">=3.10", but the
+    # docs tooling (resolve_sources.py, gen_compat_matrix.py) hard-imports
+    # `tomllib`, which is stdlib only in 3.11+. Amplify's Amazon Linux gave uv a
+    # <=3.10 interpreter (ModuleNotFoundError: No module named 'tomllib'); this
+    # forces uv to fetch/use 3.13 for both the root env and the notebook emitter
+    # project (both resolve to 3.13 locally). Applies to `uv sync` and `uv run`.
+    UV_PYTHON: "3.13"
     RUST_TOOLCHAIN: "nightly-2026-04-17" # rustdoc JSON format_version 57
     # Served at the domain root on Amplify — no subpath. Override SITE_URL
     # per-branch in the Amplify console if you want correct canonical/sitemap