Add support for finding free-threaded interpreters for --find-interpreters

[ngoldbaum]

This is partially extracted from #3113.

Currently, free-threaded interpreters are not always considered for --find-interpreters. It is included on Unix but not on Windows. This means that users need to explicitly do another build to produce 3.14t wheels. This also contributes to PyO3/maturin-action#368 and I'm planning to do a followup over there to fix that once this is merged.

With this PR, --find-interpreters always picks up 3.14t and newer. I intentionally excluded 3.13t because PyO3 0.29 will refuse to build wheels for 3.13t. This also follows manylinux, which recently dropped 3.13t from the images.

[ngoldbaum]

Spotted a minor improvement for the changelog entry

[Copilot]

Pull request overview

Adds free-threaded CPython (3.14+) to --find-interpreter auto-discovery on both Unix and Windows, and stops including the experimental 3.13t in bundled sysconfig lookups. A new FREE_THREADED_MINIMUM_PYTHON_MINOR = 14 constant centralizes the cutoff. 3.13t is still buildable when requested explicitly via -i python3.13t.

Changes:

• Introduce FREE_THREADED_MINIMUM_PYTHON_MINOR and refactor Unix candidate generation into cpython_candidate_names(), which adds python3.{minor}t entries (gated on bridge.supports_free_threaded() and requires-python).
• Add a Windows fallback that probes python3.{minor}t.exe for free-threaded Microsoft Store / PATH installs, reusing WindowsInterpreterFinder dedup logic.
• Start bundled free-threaded sysconfig lookup at 3.14 instead of 3.13, update test_lookup_target snapshots, add test_cpython_candidate_names_free_threaded, document the abi3 + --find-interpreter interaction, and add a changelog entry.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/python_interpreter/mod.rs	Adds FREE_THREADED_MINIMUM_PYTHON_MINOR = 14 constant.
src/python_interpreter/discovery.rs	New cpython_candidate_names() helper, Windows python3.{minor}t.exe fallback, updated snapshot tests, and a new unit test.
src/python_interpreter/config.rs	Uses the new constant so bundled free-threaded configs start at 3.14.
src/python_interpreter/resolver.rs	Documents that --find-interpreter cannot produce abi3 + free-threaded wheels (needs explicit -i python3.14t).
Changelog.md	Adds Unreleased entry describing the new behavior.

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[ngoldbaum]

Looks like something broke unrelated to this change yesterday. I'll try to dig in later to unbreak CI...

[ngoldbaum]

It looks like conda updated pip from 26.0.1 to 26.1.1 and 26.1.0 dropped support for Python 3.9.