add atomic global counter system

[Vizonex]

What do these changes do?

These changes are based off @devdanzin's second fuzzer for freethreading mode which noted using a global atomic counter system instead of a normal uint64_t value I did however make one small nitpick/change from that original design to just recast (_Atomic(uint64_t)*)&state->global_version which was not in the original suggestion mainly due to my code editor misbehaving on me when I would go to set it into the actual structure.

Are there changes in behavior for the user?

Just bug fixes.

Related issue number

#1321

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes

[codspeed-hq]

Merging this PR will not alter performance

✅ 245 untouched benchmarks

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_{Comparing Vizonex:global-state (8bb2038) with master (637ffa9)}

[bdraco]

Thanks @Vizonex

[Copilot]

Pull request overview

This PR updates multidict’s global version counter to be atomic (supporting CPython free-threading) and adds an isolated regression test to detect lost increments under concurrent mutation.

Changes:

• Make mod_state.global_version an atomic and update NEXT_VERSION() to use atomic_fetch_add_explicit(..., memory_order_relaxed).
• Add an isolated free-threading test that asserts the version delta matches the number of concurrent writes.
• Adjust build configuration to add C11 atomics flags for MSVC via a custom build_ext.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
multidict/_multilib/state.h	Switches the module-level global counter to an atomic and updates increment logic.
tests/isolated/multidict_global_counter.py	New isolated regression test for concurrent version increments (free-threaded builds).
tests/test_leaks.py	Registers the new isolated script in the isolated-script runner list.
setup.py	Introduces custom build_ext to apply platform-specific C compile flags (incl. MSVC C11 atomics).
CHANGES/1328.bugfix.rst	Adds a changelog entry for the atomic counter bugfix.

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[Copilot]

MSVC handling currently prefixes BASE_CFLAGS values with / (e.g. O0 -> /O0, g3 -> /g3). These aren't valid MSVC flags (/Od and /Zi//Z7 are the usual equivalents), so Windows builds with MULTIDICT_DEBUG_BUILD=1 will fail. Consider defining a separate MSVC-specific debug/release flag list instead of reusing BASE_CFLAGS verbatim.

[bdraco]

Looks like the tests still need some work