Define free-threaded extension build behavior

[nitzmahone]

• Determine which Cython major versions will be supported on PyYAML version that supports free-threading (including an upper bound on released sdists?)
• Set # cython: freethreading_compatible = True on FT builds against 3.1+ once a minimal FT test suite is in place
• Make any FT-specific build accommodations defensive/best-effort (since Cython's support is still experimental as of 3.1)
• Determine wheel matrix for FT builds - Cython 3.1 would finally let us switch to abi3, but t builds still require version-specific wheels until abi3t is settled. Consider limiting free-threaded wheels to 3.14+ and a much smaller set of arches/C-ABIs.