🧪 TEST: demonstrate file-list mode (files) in the example project

[ubmarco]

What

The tests/example reference project previously demonstrated only directory mode (dir) mounts. This adds a checked-in "release notes" bundle mounted via file-list mode (files), so the example now covers both mount modes side by side.

Changes

• New bundle tests/example/release-notes/ — index.rst + notes/{2026-q1,2026-q2,2026-q3-draft}.rst.
• docs/ubproject.toml — a files = [...] mount at _generated/release-notes, wired via attach_to like the existing mounts.
• build_docs_sandbox.sh — stages the release-notes/ tree next to docs/ in the temp working tree (mirroring the existing showcase/ staging) so the sandboxed bazel build //:docs_html build resolves the mount. The in-workspace bazel run //:build_docs path needs no change (it runs in the real source tree).
• test_example.py — asserts the three picked files render at a flat namespace (_generated/release-notes/2026-q1, not .../notes/2026-q1), that the unlisted 2026-q3-draft.rst is never mounted (the behaviour that distinguishes file-list from directory mode), and that attach_to wires the entry doc into the host toctree.
• README — documents directory-mode vs file-list-mode.

Verification

• uv run pytest -m bazel tests/test_example.py — passes (Bazel build + sphinx-build -nW).
• Both README build entry points: bazel build //:docs_html (sandboxed) and bazel run //:build_docs (in-workspace) — both render the release-notes pages, no 2026-q3-draft.html.
• Full suite: 157 passed.
• Pinned ruff@0.12.2 format/check clean; prek run --all-files all hooks pass (incl. taplo).

Note: test_example.py copies the whole example (shutil.copytree) and runs sphinx-build directly, so it does not exercise the curated //:docs_html staging — that's why the initial version passed the test but broke the README's Bazel build. Fixed here; a dedicated //:docs_html regression test is a reasonable follow-up (happy to add if you want it).

Deliberately skipped (flagging for review)

• Changelog — example/test-only, and files mode already shipped in 0.1.0, so there's no user-facing behavior change. Mirrors PR ✨ NEW: reference a pre-built HTML report from a mounted bundle (html_extra_path) #6 (the html_extra_path example), which also added no changelog entry. Happy to add one under a new unreleased section if you'd prefer.
• docs/source/ — file-list mode is already fully documented in configuration.rst; nothing behavioural changed.