Remove SAGE_LOCAL from sage.config

[cxzhong]

Fixes #42459.

sage.config contains build-time configuration, while SAGE_LOCAL is a runtime environment setting with a different meaning in Sage-the-Distro. Keeping SAGE_LOCAL in both places made the generated Kenzo path use the Python venv instead of the non-Python installation prefix.

This PR now:

• removes SAGE_LOCAL from sage.config;
• configures KENZO_FAS directly in Meson from an explicit -DSAGE_LOCAL option used by Sage-the-Distro;
• leaves KENZO_FAS empty in pure Meson builds so that ECL uses its normal search path;
• configures the ThreeJS directory directly instead of deriving it from sage.config.SAGE_LOCAL;
• keeps sage.env.SAGE_LOCAL runtime-based, with the environment taking precedence and the active Python prefix as the pure-Python fallback;
• removes obsolete runtime placeholder replacements now that Meson supplies complete values;
• adds regression coverage for both Meson modes, runtime fallback/override behavior, and fully substituted paths.

The runtime fallback restores the behavior that existed before sage.config.SAGE_LOCAL became unconditional: Sage-the-Distro still exports its non-Python prefix, while pure Python installations use their active sys.prefix.

Testing

• ./sage -t src/sage/config_test.py — 8 passed
• .venv/bin/python3 -m pytest -q src/sage/config_test.py — 8 passed
• Conda Python: python -m pytest -q src/sage/config_test.py — 8 passed
• ruff check src/sage/config_test.py
• git diff --check upstream/develop...HEAD
• pure Meson reconfiguration and runtime smoke tests with SAGE_LOCAL unset
• fresh Meson setup with -DSAGE_LOCAL="/tmp/sage 42606 local"; generated config.py compiled and its Kenzo/ThreeJS paths were verified

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

None.

[github-actions]

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[tobiasdiez]

I don't have a very strong opinion on this, but I would prefer to "just" remove SAGE_LOCAL from the config.py file. For Kenzo, it should work to look in meson for it in SAGE_LOCAL, similar to how maxima is handled.

[cxzhong]

@tobiasdiez could you review this again?