Support diagnostic text reads from the RTC device

[JaxCalder]

cat /dev/rtc previously failed because the driver rejected read operations. Return a 20-byte diagnostic line, YYYY-MM-DDTHH:MM:SS\n, followed by EOF. The line carries the device's civil time without assuming a timezone.

Capture time on the first nonempty read per open handle. Partial reads and seeks reuse the same snapshot, so a changing calendar cannot splice two timestamps together; reopen for fresh time. Unset clocks return ENODATA without caching the failure. Validate handle, read permissions, buffer and offset; setting time continues through the standard ioctl interface.

Validation: native ctest 3/3 with 14/14 driver steps, STM32F4/F7 builds, and STM32F746G-DISCO RTC 12/12 through VFS, dmdrvi 6/6, RNG 6/6. Actual cat output checked before and after restoring PC time. Snapshot/partial-read/EOF/buffer-guard/error cases are covered; console evidence and package hashes are in docs/testing.md.

This is a separate change based on main after the merge of #1.