Add file descriptor syscalls and initarfs-backed file access

[codingskynet]

Summary

• Add per-thread file descriptor tables with standard input, output, and error descriptors.
• Add open, read, write, and close syscalls.
• Introduce a two-register syscall result ABI using status and value fields.
• Add normalized absolute path handling and filesystem contexts.
• Expose console and initarfs files through a shared file-node interface.
• Add MicroPython file-reader support so modules can be imported from the initarfs.
• Update the simple userland program to exercise file open, read, and close operations.
• Fix smoke-test exit-code types and a broken Rustdoc link.

Syscall ABI

Returning syscalls now use:

• a0: status code, where 0 indicates success
• a1: return value, valid when the status is 0

The following syscalls are supported:

Number	Syscall	Arguments
0	exit	exit code
1	write	file descriptor, buffer, length
2	read	file descriptor, buffer, length
3	open	path, path length
4	close	file descriptor

Validation

• make check
• make ports-verify PORTS=micropython
• make run DEBUG=1 FEATURES=smoke
• Verified the simple userland file read flow in QEMU.
• Verified import eratosthenes from the initarfs in MicroPython.