Free-threaded tdom

[pauleveritt]

I had Claude Code do a pass at free-threaded tdom. The main thing it found: use of random.choice in placeholders. I have a branch if @ianjosephwilson and @davepeck are interested, showing:

• The analysis it did and what it found
• Fix the main issue, skip the remaining as won't-fix (we allow mutating children after construction)
• How to add pytest-freethreaded and some tests that can exercise it
• Automation in Justfile
• Performance results for 3 cases (3.14, 3.14t single threaded, 3.14t multi-threaded)

I'm not submitting as PR as I suspect you would only want it for information. Let me know if I should push the branch.