`executeCommand("undo")` is non-functional in some headless `.vscode-test`
instances (it does not restore the buffer — see #54's diagnosis), which
false-fails every undo-dependent host E2E and turns `main`'s E2E red there even
though the product code is fine.
Add a RUNTIME preflight probe (`undoCapable.ts`): edit a scratch buffer, undo,
and report whether the buffer was actually restored (memoized per run). The #38
undoMarks suite gates on it via `suiteSetup` — running normally where undo works
(full coverage preserved, real regressions still caught) and skipping with a LOUD
console warning where it doesn't (no false red, no silent loss — the skip is
logged and shows as `pending`).
This fixes the "main E2E red" symptom. The underlying headless-undo limitation is
documented in #54; #40's undo-behavior coverage runs wherever undo works.
Test-infra only — no product code changed. Verified: with undo broken locally the
#38 suite skips (1 pending) and both E2E passes exit 0.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Ben Stull