Closes #78. Fixes #156. Alternative approach to #131.

Problem

Two things have been bugging me about GLIM for a while:

1. The FAT32 4 GiB file-size limit. Ubuntu 22.04+ desktop ISOs already exceed it, and
   the trend is only going in one direction.
2. No way to have a data partition on the same stick, which would be useful for
   transferring files between live environments.

What this PR does

It adds glim-partition.sh, a new automated partitioning script, and extends glim.sh
to support a GPT multi-partition layout, while keeping the existing single-partition
FAT32 setup working exactly as before.

Default mode (simple, universal):

./glim-partition.sh /dev/sdX

Creates a single FAT32 partition (MBR), identical to what you'd set up manually today.
Works on Windows and macOS, works across BIOS and EFI. Still subject to the 4 GiB limit.

GPT mode (for large ISOs):

./glim-partition.sh /dev/sdX --gpt
./glim-partition.sh /dev/sdX --gpt --data-size 32G
./glim-partition.sh /dev/sdX --gpt --data-size 32G --data-fs ext4

--data-size implies --gpt. The GLIMDATA partition defaults to exFAT, which has no
file size limit and is readable natively on Windows, macOS, and Linux without drivers.
Use --data-fs ext4 if you only care about Linux. exFAT partitions are accessible
to all users by default when mounted. ext4 GLIMDATA is set world-writable (chmod 1777)
so any live environment can write to it regardless of UID/GID.

Why no grub.cfg changes?

grub-install --boot-directory pointing at the ext4 GLIM partition makes GRUB's $root
that partition naturally. The existing probe --set rootuuid --fs-uuid $root picks up
the correct UUID, so all iso-scan/filename=, img_dev=, and findiso= boot parameters
work without modification. Existing ISO paths are preserved.

Changes

• glim-partition.sh (new): automated partitioning script. Destructive; requires
  explicit yes confirmation and refuses to run as root or touch the system disk.

• glim.sh: detects a separate ESP via partition type GUID for EFI install;
  handles NVMe naming correctly; chowns the GLIM partition after grub-install so
  rsync and subsequent ISO copies work without sudo. Also fixes glim.sh: Escape key (and other non-'n' input) accepted as yes at prompts #156: prompts now only
  accept y, Y, or Enter as yes; Escape no longer silently proceeds.

• README.md: documents both layouts with automated and manual setup instructions.
  Includes a Secure Boot note: some ISOs (Gentoo, Arch, Artix) ship shims not signed by
  a Microsoft-trusted CA and will fail on Secure Boot systems; this is not a GLIM bug.

• tests/ (new): pytest suite that runs against real loop devices. Covers partition
  layout, glim.sh install correctness, and QEMU BIOS/UEFI boot smoke tests. No mocking.

Comparison with #131

#131 solves the same problem differently: a small FAT32 anchor partition plus a separate
GLIMISO partition for ISOs. That approach requires significant grub.cfg changes and
moves ISOs from boot/iso/ to iso/, a breaking change for existing setups. This PR
avoids both by using --boot-directory to point GRUB at the ext4 partition directly.

Tested

Tested on a 232 GB USB stick with a GPT layout and 64 GB GLIMDATA partition. Elementary
OS boots successfully via UEFI. BIOS boot confirmed working.