feat(pty): implement POSIX pseudo-terminal support

[ryanbreen]

Summary

• Adds complete PTY (pseudo-terminal) subsystem for terminal emulation
• Implements POSIX-compliant syscalls: posix_openpt, grantpt, unlockpt, ptsname
• Creates devptsfs virtual filesystem for /dev/pts/* device nodes
• Integrates sys_open to route /dev/pts/* paths correctly
• Provides libbreenix userspace API for PTY operations
• Includes integration test with byte-level data verification
• Adds telnet server skeleton for future remote shell access

Key Components

Component	Location	Description
PTY Core	kernel/src/tty/pty/	Master/slave pair management, circular buffers
Syscalls	kernel/src/syscall/pty.rs	posix_openpt, grantpt, unlockpt, ptsname
devptsfs	kernel/src/fs/devptsfs/	Virtual filesystem for /dev/pts/*
sys_open integration	kernel/src/syscall/fs.rs	Routes /dev/pts/* to devptsfs
Userspace API	libs/libbreenix/src/pty.rs	Safe wrappers for PTY syscalls
Integration test	userspace/tests/pty_test.rs	Full PTY flow with data verification

Test plan

• Build succeeds with 0 warnings
• All 226/226 boot stages pass
• PTY integration test verifies:
  • posix_openpt with O_RDWR | O_NOCTTY
  • grantpt/unlockpt sequence
  • ptsname returns valid path
  • open("/dev/pts/N") succeeds
  • Master→slave data flow with byte verification
  • Slave→master data flow with byte verification
  • Error path: grantpt rejects invalid fd

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