test: adaptive cancellation assertion races before partial transition

## Problem

The full adaptive browser gate can deterministically fail the test `cancellation during a partially failed action retains bounded non-replayable evidence` because its fixed 250ms abort fires before any error transition is recorded.

## Reproduction

```bash
npm run test:browser-accessibility-oracles
```

Observed repeatedly on current `origin/main` plus PR #2099:

```text
✖ cancellation during a partially failed action retains bounded non-replayable evidence
AssertionError: assert(failed)
at tests/browser-adaptive-exploration.test.ts:444
```

The result is already `incomplete`/cancelled, but `result.transitions.find((transition) => transition.status === \"error\")` is undefined because cancellation wins the timing race before the intercepted repeat action reaches its expected partial failure.

## Expected

The test should synchronize cancellation to the intended in-flight action state instead of using a fixed wall delay, then assert bounded non-replayable evidence deterministically.

## Scope

Test synchronization only unless investigation shows runtime cancellation dropped an already-observed transition. Do not weaken the production evidence contract.

Empieza por tests/browser-adaptive-exploration.test.ts, especialmente por la aserción fallida alrededor de la línea 444, y ejecuta npm run test:browser-accessibility-oracles para reproducir la condición de carrera. Traza la acción parcialmente fallida y el momento de la cancelación; después, sincroniza la cancelación con el estado previsto de la acción en curso en lugar de usar un retraso fijo de 250ms. La tarea estará terminada cuando la aserción de evidencia acotada y no reproducible determinísticamente pase, sin debilitar el contrato de evidencia de producción.

Escrito por el modelo de indexación a partir del texto del issue.