fix: use signer's own Free() instead of system libc free on Windows

[lavrric]

Summary

Fixes a Windows crash/heap corruption caused by cross-CRT memory freeing. The previous code used ctypes.util.find_library("c") (via libc.py) to find the system C library's free(), which returns msvcrt.dll on Windows. However, the Go signer DLL is cross-compiled with MinGW and may use a different C runtime (or statically link its own). Calling one CRT's free() on memory allocated by another CRT's malloc() is undefined behavior on Windows.

The fix replaces libc.free() with the signer's own exported Free() function, which is guaranteed to use the same allocator that C.CString() used to allocate the memory.

Requires the companion PR in lighter-go (elliottech/lighter-go#56) which exports the Free function, and the signer binaries in lighter/signers/ must be rebuilt from that updated lighter-go before this change will work.

Review & Testing Checklist for Human

• The signer binaries (.dll, .so, .dylib) in lighter/signers/ must be rebuilt from the updated lighter-go that exports Free before merging this PR. Without that, __populate_shared_library_functions will fail at the signer.Free line because the symbol won't exist.
• Verify decode_and_free is never called before get_signer() — the code references the module-level __signer directly, which is None until get_signer() runs. Current call flow looks safe but worth confirming.
• Test on Windows: install the rebuilt DLL + this Python code, and verify basic operations (e.g., GenerateAPIKey, create_order) work without crashes or heap corruption.
• lighter/libc.py is now unused dead code — consider removing it in a follow-up.

Notes

• On Linux/macOS, there's a single system libc, so the old free() happened to work. The new approach is correct on all platforms.
• The libc.py module is no longer imported anywhere but was not deleted in this PR to keep the diff minimal.

Requested by: @lavrric

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