fix: install every lazily-imported dependency in the Colab bootstrap; gate setup_colab.py on PRs

[Jammy2211]

Summary

Closes #167.

autonerves.setup_colab installs the PyAuto stack on Colab with --no-deps, so every dependency Colab does not preinstall must be named in _SHARED_EXTRAS. HowToFit tutorial 5 died on Colab on 2026-09-15 with ModuleNotFoundError: No module named 'corner' after its emcee run — the third report of this class (#166 closed it for emcee/dynesty). This PR audits the whole list rather than adding one package, and adds the PR-time gate that stops the list drifting again.

• _SHARED_EXTRAS gains corner==2.2.2, optax>=0.2.5, xxhash<=3.4.1, blackjax>=1.6.2 (lazily imported autofit deps) and corrects two specifiers drifted from PyAutoFit's pyproject (nautilus-sampler 1.0.4 → 1.0.5, anesthetic ==2.8.14 → >=2.9.0).
• A second pass driven by PyAutoHeart's new Colab gate (feat(heart): Colab gate — check F seeds from Google's manifest and audits every lazy import PyAutoHeart#228, which probes every import after the real --no-deps bootstrap in a venv built from Google's published Colab manifest) adds jax_zero_contour>=2.0.0,<3.0.0 — an autogalaxy base dependency reached by autogalaxy/operate/lens_calc.py:1452,1884 (critical curves) — and zeus-mcmc==2.5.4 (af.Zeus, run by autofit_workspace plot/zeus_plotter, cookbooks/search, searches/mcmc). Every runtime requirement of both is already shipped by Colab (jax/jaxlib 0.11.1; numpy, scipy, tqdm, setuptools, pytest, matplotlib, seaborn, scikit-learn), so the --no-deps install is clean.
• TestSpecifiersTrackAutofit derives its expectations at run time from PyAutoFit's and PyAutoGalaxy's pyproject.toml (never restating a literal it guards); test_every_project_installs_every_sampler requires the full set.
• New .github/workflows/colab-gate.yml: PRs touching setup_colab.py check out PyAutoHeart main and run check F with this checkout overlaid (COLAB_GATE_AUTONERVES_SRC), the PR-time complement of Heart's release-time verify_install_release. It goes green only once feat(heart): Colab gate — check F seeds from Google's manifest and audits every lazy import PyAutoHeart#228 is merged (Heart main must contain colab_gate.py).

Witness with this branch overlaid on Heart check F: FAIL on five modules before the second pass → F|PASS (179 s) after it; the three remaining unguarded imports (colossus, hmf, mcp) are not notebook paths and are accepted with written reasons in Heart's colab_gate.yaml.

API Changes

None. _SHARED_EXTRAS is a private module-level list; setup() and the for_<project>() entry points are unchanged. User-visible effect: Colab notebooks additionally install the packages above at bootstrap.

Shipping

Merging fixes nothing on its own — the setup cell pip-installs autonerves unpinned at run time, so the autonerves PyPI release carrying this is what reaches every published notebook retroactively. Confirm the overnight release run has settled before cutting one.

Test Plan

• python3 -m pytest -q test_autonerves: 185 passed
• Heart check F with this branch overlaid: F|PASS, 179 s (feat(heart): Colab gate — check F seeds from Google's manifest and audits every lazy import PyAutoHeart#228 leg 3)
• colab-gate.yml green on this PR after PyAutoHeart#228 merges

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