ci: allow the test workflow to be triggered manually

Twelve commits landed on main on 2026-08-06 (f8e888c7..8452a844) without a
single workflow run being created. The push webhooks reached GitHub - the
Cursor app opened a check suite on each head commit at push time - but no CI
run was enqueued, while pull_request runs on the same workflow kept working.
The repository side is clean: the workflow is active, unchanged since the
last green push run, has no path filters, no skip-ci token in any of the
twelve messages, no rulesets and no branch protection.

The same window shows GitHub-side queueing trouble independently: four runs
created between 08-05T15:37Z and 08-06T09:33Z all have run_started_at within
four seconds of 08-06T17:36:56Z, having sat queued for 8 to 26 hours against
a 10-minute job timeout.

Whatever the cause, `push` was the only trigger that could produce a verdict
on main, so losing it left no way to get one short of pushing an empty
commit. This adds the manual trigger.

.github/workflows/test.yml

@@ -5,6 +5,10 @@ on:
     branches: [main]
   pull_request:
     branches: [main]
+  # Manual trigger. On 2026-08-06 twelve commits landed on main without a
+  # single push-event run being created, and there was then no way to get a
+  # verdict on main short of pushing an empty commit.
+  workflow_dispatch:
 
 jobs:
   test: