Implement extended stdio functions (popen, tmpfile, setvbuf)

[pbalduino]

Goal

Complete stdio coverage with process pipes, temporary files, buffer control, and other missing POSIX functions.

Context

Current stdio is basic (fopen/fclose/fread/fwrite/fprintf exist), but many common functions are missing.

Definition of Done

• [ ] popen() / pclose() - Pipe to/from process
• [ ] tmpfile() - Create temporary file
• [ ] tmpnam() / tempnam() - Generate temp filename
• [ ] setvbuf() / setbuf() / setbuffer() - Control buffering
• [ ] fileno() - Get fd from FILE*
• [ ] fdopen() - Create FILE* from fd
• [ ] getline() / getdelim() - Read line with allocation
• [ ] dprintf() / vdprintf() - Print to file descriptor
• [ ] Wide I/O variants (if wide-char support added)

Dependencies

Required

• #193 - Minimal libc ✅
• #208 - Pipes (for popen) ✅

Priority

Medium

Estimated Time

4-6 weeks part-time