Move TMP/TEMP to the workspace disk on Windows CI runners

[cboos]

Part of #248; follow-up to #264.

Problem

Even with coverage scoped to the primary job (#264), Windows unit-test steps still ran ~3.5x slower than ubuntu (~555s mean vs ~161s for the same selection). An A/B experiment on real windows-latest runners (run 28715118956, 5 variants x 2 reps, full evidence in work/win-io.md) isolated the cause: TMP/TEMP default to the throttled C: OS disk, and this tmp_path-heavy suite pays its I/O prices.

variant	mean (s)	verdict
baseline	554.5	control
defender-off	557.7	no-op — real-time protection is already off on current images
defender-excl	672.7	no-op + VM variance
ReFS dev drive	296.4	helps, but dominated
TMP/TEMP → RUNNER_TEMP	150.2	ubuntu parity

Change

One early, Windows-gated step writing TMP/TEMP = RUNNER_TEMP (the workspace disk) to GITHUB_ENV, so every later step benefits (unit, TUI, browser, and dependency installs). Plus work/win-io.md recording the experiment so the step isn't "simplified away" later.

Side benefit: tmp dirs and the checkout now live on the same drive, removing the cross-drive layout that can break os.path.relpath between tmp_path and repo paths on Windows.

Expected effect

Windows unit step ~735s (pre-#264) → ~150s; Windows jobs should drop off the CI critical path. This PR's own CI run is the end-to-end verification.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved Windows CI reliability and performance by routing temporary files to a faster temporary location before install and test steps run.
• Documentation
  • Added a write-up summarizing the Windows CI performance experiment, findings, and the recommended configuration change.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 4769c74e-5411-4704-ba41-3a12f85f6b8c

📥 Commits

Reviewing files that changed from the base of the PR and between 52da5c9 and 3990b3b.

📒 Files selected for processing (2)

• .github/workflows/ci.yml
• work/win-io.md

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📝 Walkthrough

Walkthrough

This PR adds a Windows-only CI step in ci.yml that redirects TMP and TEMP environment variables to RUNNER_TEMP via GITHUB_ENV to improve Windows test runtime, and adds work/win-io.md documenting the A/B experiment and findings supporting this change.

Changes

Windows CI Temp Redirect

Layer / File(s)	Summary
CI workflow env redirect .github/workflows/ci.yml	Adds a Windows-only step that sets TMP and TEMP to RUNNER_TEMP via GITHUB_ENV before install/test/lint/type-check steps.
Experiment write-up work/win-io.md	Documents the Windows timing problem, A/B test variants and results, findings on Defender/ReFS/temp redirect impact, the resulting CI change, and future experiment notes.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Estimated code review effort: 1 (Trivial) | ~5 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: redirecting TMP/TEMP on Windows CI runners for performance.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch dev/ci-windows-temp

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[daaain]

Oh wow, this was unexpected!

[cboos]

Oh wow, this was unexpected!

Well, that was Fable continuing where Opus dropped the gauntlet ;-)

[cboos]

For the record, the builds for this commit on main:

We're still slower on Windows (4-5 min) vs. Linux (2-3 min), but both are indeed in the same league now.