fix: fall back to main when substream playback fails

[bcbetterninja]

Summary

A Hikvision NVR channel returns RTSP 404 on substream while main stream works. Linux kiosks currently retry the failed substream without trying main. This change swaps only the affected tile to its configured main stream after an explicit playback error (on the next one-second status tick), or ten seconds without video.

Fallback is remembered per display and camera for the current layout visit, logged without stream URLs, and indicated by the M badge. A real layout switch clears that display's fallback decisions and retries substream. The shared pipeline pool keeps fallback main streams warm; other displays can continue using substream. Main failures retain existing recovery behavior without oscillating between streams. No camera configuration is changed, and fallback requires distinct configured main/sub URLs.

Validation

• All 34 Rust core tests pass, including fallback eligibility, immediate errors, stall timeout, and no repeated attempt after fallback.
• git diff --check passes; commit signature verified.
• Full Linux cargo check is blocked by missing GLib/GTK/GStreamer development libraries.
• Physical kiosk playback and layout-switch recovery still need testing; opening as draft.

Independent of the unrelated layout-refresh fix in #32. This addresses Linux playback; Android already has a separate fallback implementation.

[coderabbitai]

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[bcbetterninja]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🚀

Reviewed commit: d6fec92bc3

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