daplink_flash: Add DAPLink Flash driver for I2C bridge to W25Q64JV.

[nedseb]

Summary

• Rename lib/w2564jv/ to lib/daplink_flash/ (the flash is accessed via I2C bridge, not SPI directly)
• Implement DaplinkFlash MicroPython driver for the STM32F103 DAPLink flash bridge
• Support write (append to file) and read (sector-based) operations
• Add 13 mock tests + 5 hardware test scenarios
• Add 4 usage examples, all validated on hardware
• Update README with correct API documentation

Architecture

The STM32WB55 (MicroPython) communicates with the STM32F103 (DAPLink) at I2C address 0x3B (7-bit). The F103 bridges to the W25Q64JV SPI flash (64 Mbit).

STM32WB55 --I2C--> STM32F103 (DAPLink) --SPI--> W25Q64JV
  (MicroPython)       (0x3B)                     (64 Mbit flash)

I2C Protocol

Command	Code	Description
WHO_AM_I	0x01	Device ID -> 0x4C
SET_FILENAME	0x03	Set 8.3 filename (11 bytes)
GET_FILENAME	0x04	Read current filename
CLEAR_FLASH	0x10	Erase entire flash
WRITE_DATA	0x11	Append data (30-byte chunks)
READ_SECTOR	0x20	Read 256-byte sector (requires firmware fix, see #165)
STATUS_REG	0x80	Busy flag (bit 7)
ERROR_REG	0x81	Error flags

API

from daplink_flash import DaplinkFlash

i2c = machine.I2C(1)
flash = DaplinkFlash(i2c)

flash.device_id()                    # -> 0x4C
flash.set_filename("DATA", "CSV")
flash.write_line("temperature;humidity")
flash.write_line("25.3;48.2")
flash.clear_flash()

# Read (requires DAPLink firmware fix #165)
data = flash.read_sector(0)          # 256 bytes
content = flash.read()               # full file as bytes

Examples

Example	Description
flash_info.py	Display bridge status, error register, current filename
write_csv.py	Write CSV data and read it back
read_file.py	Read and display current file on flash
sensor_log.py	Write 200 rows of sensor data, read back, compute min/avg/max

Hardware test results

WHO_AM_I: 0x4C
set_filename / get_filename round-trip OK
write_line -> file visible on USB mass storage OK
read_sector -> content matches written data OK
sensor_log: 200 rows written (4 KB), 170 parsed back with statistics OK

DAPLink firmware dependency

The READ_SECTOR command has two bugs in the upstream DAPLink firmware (argument length mismatch + hardcoded read address). A fix is submitted in steamicc/DAPLink#1, tracked in #165.

Write operations work without any firmware modification.

Test plan

• ruff check passes
• 13 mock tests pass (no regression on other drivers)
• Hardware: device_id() returns 0x4C
• Hardware: set_filename() / get_filename() round-trip
• Hardware: Write file, verify via USB mass storage
• Hardware: Read back file content matches written data
• Hardware: 200-row sensor log write + read + statistics

Closes #5
Closes #38

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