fix(clearance): poll for sw keepalive alarm confirmation

[ChechiDev]

Summary

• Adds bounded polling loop to make_sw_keepalive_checker so transient False alarm reads retry before blocking the harness
• Retries up to 6 times at 0.5s intervals when swKeepalive alarm is absent (False); stops early on True
• Preserves None as the SW-unreachable diagnostic — not retried, not misreported as alarm-missing
• Worst-case polling: 6 × 0.5s = 3.0s inside the existing 10s _DIAGNOSTIC_READ_TIMEOUT_S envelope
• No extension behavior changed; no real smoke executed

Root cause

Real smoke on v0.47.10 blocked at sw_keepalive_confirmed with sw_keepalive_alarm_present: False. The MV3 extension creates the swKeepalive alarm asynchronously in chrome.storage.onChanged; the previous single-shot CDP read raced against that creation and consistently saw the alarm absent.

Test plan

• test_false_is_retried_and_resolves_true — False retries, True terminates early
• test_false_repeatedly_returns_false_after_max_attempts — bounded loop, not infinite
• test_none_not_retried_returns_none — SW-unreachable exits on first call
• test_false_then_none_returns_none — mid-retry None is returned correctly
• All 65 existing unit tests in the two targeted suites pass
• ruff check and flake8 clean

Closes #209