test(testing): raise waitForText poll default 5000→10000ms to de-flake render tests

[myobie]

Raise waitForText poll default 5000→10000ms to de-flake render tests

The required vitest gate flaked on screenshot.test.ts > preserves 256-color codes — timed out at 5000ms waiting for text. (This is the still-open flake
after #119; my fix raced #119's merge and didn't make it in, so re-submitting.)

Why #116 didn't fix it: #116 raised vitest's testTimeout to 15000ms, but
the real 5000ms is waitForText's own internal poll timeout
(src/testing/session.ts), which #116 never touched. So #116 fixed the
true-color flake's symptom (the blunt vitest test-timeout) but left the
underlying too-tight render-poll budget — which resurfaced as the 256-color
waitForText timeout.

Fix: raise the waitForText / waitForAbsent / waitFor default
5000→10000ms, matching the explicit 10000–15000ms that render-heavy tests
already pass (scrollback-fidelity.test.ts) and staying under
screenshot.test.ts's 15000ms vitest testTimeout, so a slow wait fails with
the useful "waiting for X" message rather than a blunt vitest timeout. Only
affects failure/slow-wait latency; a matching wait returns immediately.

Verification: render suites green locally (49 tests), full suite green,
typecheck clean. The CI-load timing can't be reproduced locally, but 10000ms is
2× the budget that tripped. Test-support-library default only; no product
behavior change. (Verified green in the #119 fresh CI run before that PR merged
parity-only.)