feat(esp_tinyusb/msc): Add optional asynchronous storage IO

[omnitex]

Main idea

TinyUSB runs every class in a single task (tud_task). Without async IO, MSC READ10/WRITE10 callbacks access the storage medium inside that task. While an SD card or flash is busy, CDC, HID and the other classes on the device are not serviced.

TinyUSB 0.19 added async MSC IO (hathach/tinyusb#2967): the callback returns TUD_MSC_RET_ASYNC, and the storage access finishes later with tud_msc_async_io_done(). This PR uses that API in esp_tinyusb, so applications need no code changes:

• CONFIG_TINYUSB_MSC_ASYNC_IO runs storage IO in a dedicated worker task. The TinyUSB task keeps servicing other classes, and write errors are reported to the host.
• CONFIG_TINYUSB_MSC_ASYNC_WRITE_BEHIND (default y) accepts a write chunk into the existing per-storage buffer and writes it in the background, so the host sends the next chunk in the meantime. This keeps write throughput without extra RAM. A failed buffered write is reported on the next command to the LUN.
• Extra cost: about 4.5 KB heap, mostly the worker stack. Measured peak use of that stack is 972 of 4096 bytes (24 %).

Results (ESP32-P4, SD card, 40 MHz)

Example: 104 MB file copied from the host while CDC sends a heartbeat every 100 ms (REMOVE log commit)

	Async IO off	Async IO on
Worst CDC latency during writes	88.3 ms	1.0 ms
Heartbeat lines above 10 ms	14	0
Write rate	2.6 MB/s	2.6 MB/s

Benchmark: CDC echo latency under MSC load, async with write-behind (REMOVE CSV commit)

Extra delay per storage access	Off: p50 / p99 / max	On: p50 / p99 / max	Write MB/s, off → on
0 ms (card only)	1.69 / 21.98 / 76.3 ms	0.33 / 0.49 / 0.89 ms	2.01 → 2.93
10 ms	11.35 / 15.02 / 97.0 ms	0.33 / 0.45 / 0.54 ms	0.53 → 0.56

With async IO off, CDC latency follows the storage access time. With async IO on, it stays at the idle value of about 0.33 ms, and SD write throughput does not drop (it goes up by 46 % at 0 ms delay). The 20 ms async run had one outlier at 65 ms; I haven't investigated it yet.

Notes

• Requires TinyUSB >= 0.19.0.
• On internal SPI flash, a few stalls remain even with async IO, because flash writes and erases disable the cache on both cores. The SD card is not affected.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.