feat(extension): support LOAD by filesystem path for out-of-tree builds

[liulx20]

Summary

Allow LOAD to accept an absolute or relative path to a .neug_extension binary, in addition to the current name-based lookup under $NEUG_EXTENSION_HOME_PYENV/extension/<name>/.

This unblocks the common out-of-tree workflow:

1. NeuG as a git submodule (or sibling) to compile the extension via NEUG_EXTENSION_CONFIGS / neug_extension_load (see feat: support out-of-tree extensions via NEUG_EXTENSION_CONFIGS #745)
2. Runtime uses pip install neug (or another NeuG build)
3. Load the locally built artifact from Python without copying it into site-packages

Motivation

Today load_extension always treats the argument as an extension name and resolves:

$NEUG_EXTENSION_HOME_PYENV/extension/<name>/lib<name>.neug_extension

import neug also overwrites NEUG_EXTENSION_HOME_PYENV to the wheel/site-packages tree. So a separately built out-of-tree artifact is not found unless the user:

• copies/symlinks into the pip extension directory, or
• overrides NEUG_EXTENSION_HOME_PYENV after import to point at the extension build root

Both work, but are awkward for local development. Path-based LOAD is the natural fix.

Proposed usage

LOAD '/abs/path/to/build/release/extension/wiggle/libwiggle.neug_extension';
LOAD 'build/release/extension/wiggle/libwiggle.neug_extension';

Name-based LOAD wiggle stays unchanged for installed / CI-packaged extensions.

[github-actions]

🤖 Auto-triage suggestion: this looks like a sub-issue of #158 — External Data Support: Multi-Format, Remote Filesystem, Data Lake (confidence: medium).

Reason: The issue is about extension loading mechanism and filesystem path support, which relates to the extension system infrastructure tracked under #158, though it's specifically about the LOAD mechanism rather than remote filesystems.

Auto-applied: type → Feature; labels → extension

If this is correct, a maintainer can link it via the GitHub UI (Sub-issues → Add). If wrong, please correct — your correction is the training signal that improves this bot.

[liulx20]

Closing: wrong scope — intended issue is out-of-tree build support via NEUG_EXTENSION_CONFIGS, not LOAD-by-path.