Implements a venv linker

[arcanis]

Make islands able to link Python dependencies into their own venv. Still experimental (for example it doesn't yet supports package conditions during resolution).

---

Note

Medium Risk
Adds a new island linker that mutates workspace filesystem state under .venv and changes local-path fetching to accept absolute paths, which could affect install/link behavior across platforms. While scoped to islands, it touches core linking/fetcher code paths and introduces new resolution constraints (one version per ident) with Unsupported errors on conflicts.

Overview
Adds a new per-island venv linker that installs an island’s resolved (non-workspace) dependency set into each workspace’s .venv/lib/site-packages by symlinking local packages or extracting cached zips, and wires it into the main linking pipeline.

Introduces a new yarn python command that mirrors yarn node but, when run from a workspace in a venv island, sets VIRTUAL_ENV, prepends .venv bin/Scripts to PATH, and prepends the workspace site-packages to PYTHONPATH (with python3 fallback).

Updates config (schema.json + IslandLinker) to accept linker: "venv" for islands, and adjusts folder:, tarball:, and link: fetchers to treat absolute paths as already-resolved (instead of always resolving relative to the parent package). Adds acceptance tests and fixtures validating venv installs, mixed PnP+venv workspaces, and link/tarball/folder sources for Python imports.

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[arcanis]

@cursor review

[github-actions]

⏱️ Benchmark Results

gatsby install-full-cold

Metric	Base	Head	Difference
Mean	2.621s	2.636s	+0.56% ⚠️
Median	2.613s	2.622s	+0.37% ⚠️
Min	2.536s	2.556s
Max	2.705s	2.793s
Std Dev	0.038s	0.058s

📊 Raw benchmark data (gatsby install-full-cold)

Base times: 2.579s, 2.626s, 2.609s, 2.601s, 2.595s, 2.634s, 2.613s, 2.676s, 2.603s, 2.621s, 2.585s, 2.615s, 2.596s, 2.630s, 2.612s, 2.603s, 2.599s, 2.584s, 2.604s, 2.613s, 2.607s, 2.536s, 2.657s, 2.686s, 2.626s, 2.614s, 2.625s, 2.675s, 2.705s, 2.703s

Head times: 2.793s, 2.767s, 2.640s, 2.602s, 2.630s, 2.582s, 2.619s, 2.604s, 2.606s, 2.624s, 2.629s, 2.597s, 2.667s, 2.635s, 2.660s, 2.620s, 2.774s, 2.582s, 2.633s, 2.621s, 2.639s, 2.609s, 2.582s, 2.556s, 2.603s, 2.628s, 2.585s, 2.594s, 2.679s, 2.709s

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gatsby install-cache-and-lock (warm, with lockfile)

Metric	Base	Head	Difference
Mean	0.374s	0.380s	+1.55% ⚠️
Median	0.369s	0.377s	+2.11% ⚠️
Min	0.361s	0.367s
Max	0.458s	0.426s
Std Dev	0.019s	0.011s

📊 Raw benchmark data (gatsby install-cache-and-lock (warm, with lockfile))

Base times: 0.418s, 0.377s, 0.370s, 0.370s, 0.368s, 0.361s, 0.364s, 0.367s, 0.367s, 0.370s, 0.363s, 0.371s, 0.369s, 0.363s, 0.458s, 0.374s, 0.368s, 0.377s, 0.372s, 0.373s, 0.367s, 0.374s, 0.366s, 0.369s, 0.364s, 0.364s, 0.373s, 0.376s, 0.367s, 0.372s

Head times: 0.371s, 0.371s, 0.367s, 0.378s, 0.370s, 0.402s, 0.377s, 0.377s, 0.369s, 0.426s, 0.371s, 0.387s, 0.392s, 0.381s, 0.375s, 0.379s, 0.379s, 0.376s, 0.377s, 0.383s, 0.385s, 0.379s, 0.381s, 0.378s, 0.381s, 0.377s, 0.374s, 0.372s, 0.374s, 0.378s

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

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^{Reviewed by Cursor Bugbot for commit 4f7e244. Configure here.}

[cursor]

Inconsistent absolute path check breaks Windows Link references

Low Severity

The fallback handler for Link references when package_data is None uses params.path.starts_with('/') to detect absolute paths, which is Unix-specific. The fetchers updated in this same PR (link.rs, folder.rs, tarball.rs) all use Path::is_absolute() to correctly handle both Unix and Windows absolute paths. If this fallback path is reached on Windows (e.g., a Link reference with a C:\... path whose package data is missing), the check would fail and the code would hit unreachable!(), crashing the process.

 

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