Fix POSIX syscall handling for consistency

[HeatCrab]

The heap allocation syscall had an implementation incompatible with the kernel's fixed-size heap design, changed to a proper stub to match other unsupported POSIX syscalls. Also added file descriptor validation in I/O syscalls to reject unsupported values beyond standard streams.

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Summary by cubic

Align POSIX syscall behavior with the fixed-heap kernel by stubbing sbrk and tightening I/O file descriptor validation. This prevents unsupported memory growth and rejects non-standard FDs in read/write.

• Bug Fixes
  • Replace _sbrk with a stub that returns -1 and sets ENOMEM to match unsupported syscall handling.
  • Validate _read/_write file descriptors to only allow 0–2; invalid values return EBADF.

^{Written for commit 7baf078. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

[jserv]

Thank @HeatCrab for contributing!