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.

Start with tests/browser-adaptive-exploration.test.ts, especially the failing assertion around line 444, and run npm run test:browser-accessibility-oracles to reproduce the race. Trace the partially failed action and cancellation timing, then synchronize cancellation with the intended in-flight action state rather than a fixed 250ms delay. Done means the bounded non-replayable evidence assertion passes deterministically without weakening the production evidence contract.

Written by the indexing model from the issue text.