README: document scripts/f3-drive-test-macos.sh

[sbrunner-atx]

Follow-up to #284, as requested: the script was merged without a README entry.

This adds scripts/f3-drive-test-macos.sh to the Bash scripts section, in the same
form as the existing entries, and notes what makes it different from them:

• it takes a disk identifier rather than a mount point, and refuses internal disks
  and partitions;
• it records the USB location ID and serial number alongside each run, so the
  physical drive, the volume, and the port match up in the log;
• the cooldown and repeat-read options exist for drives that pass a write but fail
  a read under sustained I/O, which is what the script was originally written to
  investigate;
• unlike the other scripts, it does not assume f3write and f3read sit next to it —
  it looks in PATH first and falls back to build/ — so the closing sentence
  of that section now carries an exception.

Documentation only; no code changes.

[sbrunner-atx]

While this is open, a correction to something I wrote in #284.

In that thread I reported that the USB serial number prefix 0401 correlated with the failing drives in my test batch. That claim was wrong, and I would rather it not sit uncorrected in a merged PR. A fifth drive carrying the same 0401 prefix subsequently passed a full write and two clean read passes, so the serial prefix does not predict anything.

What did hold across all five drives was the reported media name and the reported capacity in bytes — the two working groups differed on both, consistently. That is the part worth keeping.

Apologies for the noise; the script itself is unaffected, since none of this was ever encoded in it.