Preserve subprocess_create failure error details

[sheredom]

Fixes #50.

This keeps subprocess_create* success behavior unchanged (0 means success), and gives non-zero returns a documented portable meaning via the new enum subprocess_error_e:

• subprocess_error_not_found
• subprocess_error_permission_denied
• subprocess_error_no_memory
• subprocess_error_invalid_options
• subprocess_error_invalid_environment
• subprocess_error_pipe
• subprocess_error_spawn
• subprocess_error_unknown

It also preserves the platform-specific failure detail so callers can report richer diagnostics:

• POSIX: errno is restored after cleanup.
• Windows: GetLastError() is restored after cleanup.

Added docs and a regression test for a missing executable returning subprocess_error_not_found while preserving ENOENT / ERROR_FILE_NOT_FOUND.

Local verification:

git diff --check
cmake --build build
(cd build && ./subprocess_test && ./subprocess_mt_test)

Both test binaries pass locally: 48/48 each.

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