ESP32 and embedded runtime hardening

[mariusandra]

Summary

Split out of #257 (FrameOS Cloud link). Device runtime, firmware pipeline, and embedded frontend workflows:

Device runtime (Nim + ESP-IDF glue)

• Streaming decode of multi-MB PNGs and chunked HTTP bodies — downloads are no longer size-limited by socket fragmentation
• Image download failures paint an error frame instead of "No image provided"; render errors paint into the live canvas
• Frame-sized images are not pinned in the scene node cache; OOM-aborted renders that leak the heap beyond recovery trigger a restart
• Serialized scenes.json writers; console polls sleep a full tick
• Documented the no-image-proxies-for-frames rule (AGENTS.md, ESP32 README, code comments)

Firmware pipeline (backend)

• Content-addressed firmware download URLs with no-store responses
• Stale firmware cache invalidates on source changes
• Reliable OTA acknowledgement; the state partition survives re-flashing

Frontend (USB / flasher)

• USB serial render lifecycle events with Pi event parity; post-deploy USB preview retries until the first render exists
• USB commands fall back to HTTP when the port is stale; boards that re-enumerate among identical devices require explicit reselection
• Flasher keeps progress visible after the inline-controls move; reliable post-flash reboot and reconnect

Test plan

• pytest app/api/tests/test_embedded_firmware.py app/api/tests/test_embedded_device.py — 61 passed
• nim c -r src/frameos/tests/test_embedded_image_url.nim and test_interpreter_errors.nim — all green
• pnpm --dir frontend run build — typecheck + build pass

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