Fix ESP32-S3 show startup and serial transfer

[stevenmunn312-tech]

Summary

• configure DTR and RTS before opening the SD-transfer serial port, preventing Windows from resetting ESP32-S3 native USB into ROM download mode
• close partially opened serial candidates safely on retry and cover control-line state with a regression test
• correct the ESP32-S3 N16R8 profile to DIO flash + OPI PSRAM (dio_opi)
• use board_build.memory_type, the override honoured by both PlatformIO and fbuild; the older nested key made fbuild silently link dio_qspi
• prime ESP32-audioI2S local-file decoding before expensive first-frame rendering can trigger its 2.5-second header timeout
• surface audio-library diagnostics, report elapsed milliseconds instead of PSRAM read-ahead bytes, and dispatch the final show boundary at EOF

Verification

• backend: python -m pytest backend/tests — 159 passed
• frontend: npm test — 2,439 passed, 11 skipped
• production: npm run build — succeeded
• compiler database explicitly selects esp32s3/dio_opi
• hardware build and full flash on ESP32-S3 N16R8 via COM3 succeeded
• UART boot: DIO boot succeeds, 8 MB PSRAM detected, no quad_psram line-mode error
• SD mounts and the transferred 36.6-second MP3 reaches stream ready, decodes 44.1 kHz stereo audio, advances show events throughout playback, and reaches a clean EOF
• the previous ROM watchdog reset loop and zero-position audio stall are gone