fix(boot): fix bugs found in a review of the bootloader

[azenla]

A review of the bootloader turned up five bugs, each fixed in its own commit.

• Command-line arguments lost their backslashes, so --config=\path\to\config.toml loaded the wrong path. The splitter now keeps them, and the shlex dependency is gone.
• A unified kernel image with a boot counter was renamed before it was chainloaded, so it was loaded by its old name and failed. It is now booted by its renamed file.
• A one-shot entry was dropped when the menu was forced outside strict mode. It now stays the default.
• Actions of generated entries were stamped before variants and entry values were applied, so those values could not override global ones. They are now stamped when the entry is booted.
• Values that refer to other values were left unresolved in paths such as bls.path and driver paths. Stamping now repeats until the text settles, within the existing limits.

Builds and clippy pass on both targets, and the bls and parsing tests pass. I have not booted these changes in QEMU.

Not fixed here: the version comparison of 1.0 against 1.a (an existing test pins the current result), the graphical menu countdown that stalls on spurious firmware wakes, and a few cosmetic or edge-case issues.