PTY snapshot suite: rare non-repeating case failures under parallel load (pass in isolation)

## Summary

Running the full `migration*` PTY snapshot suite in parallel occasionally fails a small, **non-repeating** set of cases (~0.4% of case-executions). Every affected case passes deterministically when run in isolation, and the failing names differ between runs — so this looks like load-sensitivity in the PTY runner environment rather than any individual fixture or product bug. Filing per m0g3r's suggestion in #2483 after it showed up there first.

## Observations (macOS arm64, M-series, `just snapshot-test migration`)

Five full-suite runs across two commits, all with a fresh `packages/cli/dist` (freshness guard green):

| run | commit | result | failed cases |
|---|---|---|---|
| A | `31163c5` (PR #2483 branch) | 151/154 | `migration_dynamic_oxc_configs`, `migration_framework_shim_vue`, `migration_standalone_yarn4_idempotent` |
| B | `cc20535d` (main) | 159/161 | `migration_not_supported_vitest3`, `migration_from_tsup_monorepo_success` |
| C | `cc20535d` (main) | 161/161 | — |
| D | `cc20535d` (main) | 161/161 | — |
| E | `31163c5`, earlier same-day | 148/154 | different set again (incl. `migration_husky_or_prepare`, `migration_standalone_bun_install`) |

- **No case name repeats across runs.** Union of failures over five runs: 10+ distinct fixtures, each failing exactly once.
- **Every one of them passes in isolation** (`just snapshot-test `), immediately after the failing run, unchanged tree.
- The failure-heavy runs were the **first suite run after a fresh build / cold caches**; warm re-runs (C, D) were fully green. Cold-state work on first touch (managed runtime provisioning, registry-bridge first hits) overlapping with ~14 parallel PTY cases is my best guess at the mechanism, but I have not isolated it.
- m0g3r additionally verified in #2483 that the flaked fixtures are structurally unremarkable (step counts mid-pack; one of 21 double-`vp migrate` fixtures).

## Why it may matter

Locally it's a shrug; in CI a 0.4% per-case flake across ~650 cases makes a meaningful fraction of runs red for reasons unrelated to the diff under test, and the changing names make it hard for contributors to tell signal from noise (it cost a round of triage on #2483).

## Environment

- macOS arm64 (Darwin 25.2), 14 threads reported by the runner
- `just snapshot-test migration` (both flavors), toolchain per `rust-toolchain.toml`

Happy to re-run with any added instrumentation (timings, per-case retries, runner verbosity) if that helps narrow it — I can reproduce roughly one flaky run per two or three cold full-suite runs.

Commencez par `just snapshot-test migration` et le PTY snapshot runner, en utilisant `rust-toolchain.toml` et les fixtures de migration indiqués comme surface de reproduction. Comparez les exécutions parallèles avec un cache froid aux exécutions isolées et avec un cache chaud, en ajoutant l’instrumentation suggérée pour le minutage, la verbosité ou les nouvelles tentatives. Le travail est terminé lorsque les échecs sensibles à la charge sont expliqués et que la suite de migration ne produit plus d’échecs intermittents inexpliqués.

Rédigé par le modèle d'indexation à partir du texte de l'issue.