fix(strategy27): retain chart history across gateway failures

[jackhai9]

A validated Redis 503 during live polling previously stopped Strategy 27 and erased every ordinary marker. A single failed marker repair also cleared unrelated, verified history.

Version 0.4.3 retries validated 503 responses after the existing two-second delay without changing the live cursor or clearing history. Terminal protocol and drawing failures still stop polling, but suspend pending presentation while retaining verified markers and panel records until normal expiry. A userscript reconnect menu starts a fresh snapshot recovery. Aborted requests cannot publish late connection state or payloads.

Validation:

• Full test suite: 834 passed.
• Regression tests first failed on the previous implementation, then passed for source and generated entrypoints: 503 history preservation and cursor continuation, repair failure isolation, retention expiry, explicit restart, cancellation of late creation/repair, and terminal authorization errors.
• Strategy 27 build, all Binance syntax checks, release metadata contract, and git diff --check passed.
• Live Binance rendered UI and loaded-source verification are outstanding because the existing tab is owned by another Codex task. No trading actions were performed.