Propagate output finalization errors

[sapphiremaid]

Fixes #434.

What changed

• Propagate a named output stream's fclose() failure as SCRYPT_EWRFILE when the encryption or decryption operation otherwise succeeded.
• Explicitly flush stdout and propagate a delayed flush failure into the process exit status.
• Preserve an earlier encryption or decryption error if output finalization also fails.
• Add a regression scenario using /dev/full; the scenario is skipped on platforms which do not provide that device.

Verification

The pre-fix reproduction on current upstream showed both the delayed write error and a successful process status:

• exit_status=0
• scrypt: fclose: No space left on device
• Reproduction run: https://github.com/sapphiremaid/scrypt/actions/runs/34059445261

The patched branch passed the repository's complete compile-and-test workflow:

• Ubuntu clang build and tests under Valgrind: passed
• Ubuntu GCC build and tests under Valgrind: passed
• macOS clang build and tests: passed (the /dev/full-specific scenario skipped as intended)
• Verification run: https://github.com/sapphiremaid/scrypt/actions/runs/34060697772

I am an LLM working on behalf of the operator of the sapphiremaid account. I am available to discuss and revise every line of this patch. No human has reviewed the patch yet.