Add a Nix page; document the niri keys and the shortcut palette
Nix.md covers both halves of the optional Nix layer, and says up front which is
which because they share a language and nothing else:
- Nix + Home Manager beside pacman, for a declarative USER environment. Leads
with what it is NOT: this is not NixOS, pacman still owns the system, and
"declare the whole machine and rebuild it" is a different product.
- facts.nix as the bridge — what the machine actually is, probed rather than
remembered, so one home.nix reacts to the box instead of being edited per
box.
- Install profiles: syn-install --config, in Nix or key=value. Semantic keys
rather than menu numbers, unused keys reported by name, and the destructive
confirmations spelled out one at a time.
The two traps get their own headings because both fail silently: managing
synuirc from home.nix makes the control panel revert instead of save (Home
Manager's files are read-only store symlinks; synui writes that file live), and
/etc/synapseos/nix must not be a git repo (flakes copy only tracked files, so a
fresh facts.nix evaluates as missing).
Keybindings was missing four real binds — Super+/ (and Super+?), the palette
that lists everything else, and Super+, / Super+. , the two column moves that
are the whole point of the niri layout. Found by running synui's new
tests/readme_binds.sh against this page instead of the README; it now passes
against both.
The-Desktop said four layouts and named four: niri is the fifth in the Super+Tab
cycle. It also said the layout "is not remembered across a restart", which
layout_state_save() and tests/layout_persist.sh have both contradicted for a
while — it is per desktop and it persists.
Page and anchor links checked.