build: enable free-threaded wheels in cibuildwheel

[art049]

Opt cibuildwheel into building free-threaded CPython wheels so the
dist_instrument_hooks native extension actually ships for 3.13t / 3.14t /
3.15t.

cibuildwheel does not build free-threaded wheels by default, even when the
build pattern matches cp*. Without the opt-in, users on free-threaded
interpreters fell back to a pure-Python install with no compiled extension and
saw at runtime:

Failed to initialize instrument hooks: Failed to load instrument hooks
library: cannot import name 'dist_instrument_hooks' from
'pytest_codspeed.instruments.hooks'

Pairs with #120 (which declares the extension free-thread safe so the GIL stays
disabled on import).

[codspeed-hq]

Merging this PR will not alter performance

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 30 improved benchmarks
❌ 13 regressed benchmarks
✅ 234 untouched benchmarks

Warning

Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_open_close_fd[10]	99.2 µs	93.2 µs	+6.45%
⚡	WallTime	test_fs_write[100]	14.3 µs	13.8 µs	+3.73%
⚡	WallTime	test_sum_of_squares[sum_of_squares_sum_labmda_product]	295.6 µs	281.2 µs	+5.12%
⚡	WallTime	test_make_bandpass	5.7 µs	5.5 µs	+2.21%
⚡	WallTime	test_make_highshelf	8 µs	7.8 µs	+2.6%
❌	WallTime	test_generate_all_permutations[sequence1]	162.6 µs	167.1 µs	-2.65%
❌	WallTime	test_generate_all_subsequences[sequence1]	95.5 µs	98.7 µs	-3.2%
❌	WallTime	test_minimax[scores0]	7.7 µs	8 µs	-3.05%
⚡	WallTime	test_mmap_operation[40960]	49.9 µs	48.6 µs	+2.64%
⚡	WallTime	test_multiprocessing_map[100]	56.9 ms	55.8 ms	+2.07%
❌	WallTime	test_noop_pass	498.2 ns	538 ns	-7.39%
❌	WallTime	test_array_alloc[100]	1.1 µs	1.2 µs	-3.57%
⚡	WallTime	test_generate_all_combinations[5-4]	11.1 µs	10.4 µs	+7.02%
⚡	WallTime	test_sudoku[initial_grid0]	8.3 µs	8.1 µs	+3.52%
⚡	WallTime	test_make_lowpass	5.8 µs	5.3 µs	+9.17%
⚡	WallTime	test_iir_filter_process	3.2 µs	3.1 µs	+4.04%
⚡	WallTime	test_make_peak	7.1 µs	6.4 µs	+9.93%
⚡	WallTime	test_open_close_fd[1000]	9.4 ms	9 ms	+4.21%
❌	WallTime	test_fs_write[10000]	42.5 µs	44.2 µs	-3.95%
⚡	WallTime	test_hostname_resolution[localhost]	125.5 µs	122.2 µs	+2.74%
...	...	...	...	...	...

ℹ️ Only the first 20 benchmarks are displayed. Go to the app to view all benchmarks.

Tip

Investigate this regression by commenting @codspeedbot fix this regression on this PR, or directly use the CodSpeed MCP with your agent.

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_{Comparing fix/freethreaded-wheels (8b581dd) with master (080d620)}