Target Colab Julia 1.12.6 for notebooks

[bernalde]

Summary

• Retarget Julia notebook defaults, manifests, notebook metadata, docs, and CI smoke setup to Julia 1.12.6, matching the current Colab runtime.
• Add IJulia as an explicit notebook/sysimage environment dependency so Julia extension packages load under the active notebook project.
• Keep the Colab version-mismatch fallback as a future guardrail, but skip warm package loading after a session-only resolve/update so users can restart before loading the refreshed graph.
• Align smoke scripts with the notebook warmup path and remove Julia 1.12 world-age warning noise in the notebook 5 cache smoke.

Tests run

• Passed: make verify-julia-colab-smokes
• Passed: make verify-julia-colab-mismatch-smoke
• Passed: make verify-julia-notebook5-cache-smoke
• Passed: make verify-julia-colab-notebooks COLAB_JULIA_NOTEBOOKS=notebooks_jl/3-GAMA.ipynb
• Passed: UV_CACHE_DIR=.uv-cache uv run --group docs python -m unittest discover -s tests (65 tests)
• Passed: python -m py_compile scripts/verify_colab_runtime_smokes.py scripts/verify_notebooks.py
• Passed: git diff --check

Notes

• I did not run every Julia notebook end to end in hosted Colab. The GAMA notebook was executed locally through the Colab-style verifier, and all Julia notebook environments were covered by mismatch/instantiate smokes.

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[bernalde]

1. Blocking issues: none found.
2. Nonblocking issues: none found.
3. Questions: none.
4. Tests run and outcomes:
   • UV_CACHE_DIR=.uv-cache uv run --group docs python -m unittest discover -s tests passed, 65 tests.
   • make verify-julia-colab-smokes passed for 3-GAMA, 4-DWave, and 5-Benchmarking import/bootstrap smokes.
   • make verify-julia-colab-mismatch-smoke passed, including all Julia notebook project instantiate checks.
   • make verify-julia-notebook5-cache-smoke passed.
   • make verify-julia-colab-notebooks COLAB_JULIA_NOTEBOOKS=notebooks_jl/3-GAMA.ipynb passed.
   • python -m py_compile scripts/verify_colab_runtime_smokes.py scripts/verify_notebooks.py passed.
   • git diff --check origin/main...HEAD passed.
   • GitHub checks inspected: build passed, julia-notebook-smokes passed, deploy skipped as expected for a PR.
5. Merge recommendation: merge-ready as-is.