ci: let rstack-ecosystem-ci run the E2E suites

[fi3ework]

Summary

Two pieces that rstack-ecosystem-ci needs to run this repo's E2E suites against an unreleased
@rstest/core on ubuntu-latest (#40). This repo's own CI stays Windows + macOS.

 e2e/run.mjs
+  linux && !DISPLAY → re-exec self under `xvfb-run -a`
 e2e/setupFixtures.mjs
+  RSTACK_E2E_TOOLCHAIN_OVERRIDES="name=<absolute pkg dir>,…"   (malformed entry → throw)
+    pack → <fixture>/.toolchain/<name>.tgz
+    fixture pnpm-workspace.yaml: overrides { name: file:.toolchain/<name>.tgz }
+    per fixture: fail if pnpm-lock.yaml still holds name@<registry version>
+    per run: fail if no fixture resolved any override through a tarball

eco-ci's override goes into the consumer's root pnpm-workspace.yaml, but every E2E fixture is its own workspace root, so it never reaches them. The override therefore lives in each fixture's yaml and points at a tarball inside the fixture by relative path: the dependency-recovery suites copy a fixture to the OS tmpdir and reinstall with --frozen-lockfile, and only a fixture-relative file: spec survives that copy. Fixtures that do not depend on the package (e.g. rslint) are no-ops. The yaml is modified in place and not restored: the env is meant for disposable checkouts (ecosystem CI); locally, git checkout -- packages/vscode/e2e undoes it.

Evidence

• Before: fixtures always install the pinned registry version; Linux had no display path.
  After: local runs on macOS with VSCODE_CLI=1 and RSTACK_E2E_TOOLCHAIN_OVERRIDES="@rstest/core=<rstest main checkout>/packages/core":

  [e2e] rstest-workspace-1: @rstest/core resolved through .toolchain (no registry version in pnpm-lock.yaml)
  [e2e] rstack: @rstest/core resolved through .toolchain (no registry version in pnpm-lock.yaml)   # transitive via rstack
  [e2e] rslint: @rstest/core not in graph, override unused
  [e2e] running slice: rstest   → Exit code: 0, Exit code: 0 (incl. dependency-recovery)
  [e2e] running slice: vscode   → Exit code: 0, Exit code: 0
  

  git status --porcelain after the rstest run (nothing untracked; .toolchain/ is ignored), then empty after git checkout -- packages/vscode/e2e:

   M packages/vscode/e2e/fixtures/rstack/pnpm-workspace.yaml
   M packages/vscode/e2e/fixtures/rstest-ownership/pnpm-workspace.yaml
   M packages/vscode/e2e/rstest/fixtures/workspace-1/pnpm-workspace.yaml
   M packages/vscode/e2e/rstest/fixtures/workspace-2/pnpm-workspace.yaml
  

  Error paths:

  $ RSTACK_E2E_TOOLCHAIN_OVERRIDES=@rstest/core node e2e/setupFixtures.mjs rstest
  Error: RSTACK_E2E_TOOLCHAIN_OVERRIDES: expected name=<absolute package dir>, got @rstest/core
  $ RSTACK_E2E_TOOLCHAIN_OVERRIDES=@rstest/core=<dir> node e2e/setupFixtures.mjs rslint
  Error: RSTACK_E2E_TOOLCHAIN_OVERRIDES: no fixture in this run resolved @rstest/core through a tarball; the override did nothing
  

  Smoke without the env: [smoke] OK, exit 0.
  xvfb evidence: three-platform run on the rebased branch, https://github.com/rstackjs/rstack-editor/actions/runs/38038161388 (E2E ubuntu-latest pass).

Merge Danger

Door: two-way

Blast Radius: E2E setup only (e2e/setupFixtures.mjs, e2e/run.mjs xvfb re-exec, CI comment); no extension source

[fi3ework]

Closing without merging: the ecosystem-CI E2E suite this prepared for is not going ahead.

rstack-ecosystem-ci now runs this repo's test:unit + lint (type-checked) on every rstest main push (rstackjs/rstack-ecosystem-ci#99), and Renovate's rstack toolchain group opens a PR with the three-platform E2E on every release. That matches the protection rstack-cli has, and the remaining signal (a runtime-only break in a real VS Code, caught by neither type-check nor unit tests) did not justify keeping the fixture-injection code in setupFixtures.mjs plus ~8 min per upstream push.

Both commits (xvfb re-exec, RSTACK_E2E_TOOLCHAIN_OVERRIDES) only serve that suite, so neither lands. The branch stays as a reference if the decision is revisited.