docs: virtiofs re-measurement script and honest 3.0.1 status (#523)

[zcsizmadia]

Refs #523 (does not close it: the 9p baseline is still to be taken).

Items 1 and 2 of #523 landed in #528. Item 3, re-measuring the docs/vm-sizing.md table, is blocked on a 9p baseline from the same machine and WSL build, which needs wsl --shutdown, so it can't be done from a machine where that is unsafe.

What this adds

• scripts/measure-mounts.ps1: the page's method (Windows folder bind-mounted into a container, timed inside it, cold first write discarded, 3 rounds, medians with range, machine and WSL build recorded).
  • Measures whichever transport the machine is on and never runs wsl --shutdown.
  • -Expect 9p|virtiofs is a guard: it reads the live /proc/mounts and refuses to record when it disagrees (setting wsl.virtiofs without the shutdown leaves you on 9p and looks like success).
  • -Compare 9p.json,virtiofs.json prints the page's table with ratios, and refuses runs from different WSL builds or machines.
• docs/vm-sizing.md: states the table is WSL 2.9.11 and that no claim has been confirmed on stable WSL; records the one-sided 3.0.1 virtiofs numbers from the issue explicitly as giving no ratio (including that ls -l is not faster than the old 0.22 s, still unexplained); documents how to take the baseline.
• doctor's virtiofs detail now says the ratios are not yet re-confirmed on 3.0.

Testing

• -Compare checked against synthetic fixtures (ratios, same-transport and mismatch refusals); the -Expect guard checked against a missing distro.
• The measuring loop was smoke-tested once end to end through a Docker Desktop container on WSL 3.0.1 with -Rounds 1; those numbers are discarded and not recorded anywhere. It has not been run against a Skrog engine.
• No numbers in the table were changed.

Still open on #523

Run the script on virtiofs and on 9p (same machine, stable WSL), paste the -Compare output into the table, update the "2.9.11" wording with the WSL version and date, decide whether 4.0× / 3.1× hold, and look at ls -l.