Run pants under free threaded python

[tobni]

Switches Pants' own runtime to free-threaded CPython and makes the engine and
Pants' Python code thread-safe without the GIL.

This has been validated as running without error in my companies CI, and nets great speedups.

Most important callouts:

• Keep a persistent per-thread PyThreadState in the executor. Without
  this, every Python::attach cycle tears down the thread state, and on 3.14t
  that abandons the thread's mimalloc heaps. Created per tokio thread, torn down in on_thread_stop (blocking
  threads are recycled, so the state must not leak per thread creation).
• Makes run-scoped internals thread-safe: rule_visitor, exception_sink, logging, build_root, run_tracker.
• PBS does not ship debug-symbol stripped binaries for free threaded python, which means pants-scie downloads a fatter artifact. Fooled by stale validation / docs. Stripped, free threaded PBS builds are available, but requires upstream changes in lift (Accept freethreaded install_only PBS flavors a-scie/lift#200) and pex (no PR but john likely only needs signs of life on our end as per Upgrade to Python 3.14 #22769 (comment).

I used #23437 as a launching point.

I have on speculation set this as shipping in 2.34.x, as I believe 2.33 is too far along the dev-cycles. But I also do not see any reason to delay further.

Blocks pantsbuild/scie-pants#541

[cburroughs]

(Some adjacent or related issues #22790 #22946 #15840 #21518 so I don't need to search for them again.)

@tobni As I understand it there isn't a stable ABI for free threaded until 3.15. Having dug in to do this work, do you have a sense of how much of a problem or risk that is (or is not)?

[tobni]

@cburroughs Risk is close to none afaik, it is mostly a pyo3 compat (meaning we couldnt build it) issue. If we are allowed to assume pants is ran as a scie we defined, the internal 3rd party lockfile does the rest. I think that is ok, building a pants dist and targeting a different python would be highly special usecase.

If a plugin uses native deps in rule code that could bite them iff there is no 3.14t wheels for them.

[sureshjoshi]

Pretty substantial (and consequential) change - so I tacked on a bunch of reviewers