feat(linux): support firmware preparation on Linux

[phakoda]

Goal

Make Linux a first-class host for the current darwin-vm root-shell workflow, so firmware preparation no longer requires a Mac.

What changed

• add dmgutil.sh with macOS (hdiutil/ditto) and Linux (linux-apfs-rw/cp) backends
• make get_files.sh run on Linux and use ldid for ad-hoc signing/CDHash extraction
• make fix_perms.sh run on Linux and write numeric ownership 0:0
• add LINUX.md with the Linux prerequisites and full prep/build/boot flow
• add generated artifacts to .gitignore

Provenance / rebase notes

This is based on the Linux-host implementation from jprx#1, which reports end-to-end get_files.sh -> fix_perms.sh -> run.sh testing for both iPhone17,3 and Mac16,10 guests on Ubuntu 24.04 x86_64.

It is rebased onto the current phakoda/darwin-vm main (9044efc) and intentionally does not import that PR's unrelated SEP/device-tree or kernelcache log-suppression changes. That keeps the newer pre-SPTM InvalidateHmac fix on current main untouched.

Validation

• bash -n dmgutil.sh get_files.sh fix_perms.sh passes in a Linux environment.
• The branch is exactly one commit ahead of current main and zero commits behind.
• Full firmware/boot execution cannot be reproduced inside the current automation shell because it has no outbound GitHub network access and no privileged linux-apfs-rw kernel module; the upstream PR provides the end-to-end Linux boot evidence this port is based on.

Scope

This establishes the Linux host foundation. Display/GPU, HID/input, networking/Wi-Fi, Bluetooth, GUI services, and SpringBoard remain subsequent emulation milestones.