docs: add TROUBLESHOOTING.md for the five common rebuild errors

Covers: option-already-declared (duplicate mkOption), attribute-missing
(forgot to import nomarchy.nixosModules.system), Stylix target conflict
(needs lib.mkForce, not bare bool), home-manager .hm-bak churn (left over
from backupFileExtension after first install), and impermanence path
missing (dir not in environment.persistence list).

Each entry has the literal error text, the cause, and a copy-paste fix.
Linked from README.md and docs/MIGRATION.md so users hit it before
guessing.

docs/ROADMAP.md

@@ -28,7 +28,6 @@ Guardrails (apply when adding anything):
 - **Gaming — declarative flathub remote.** `services.flatpak.enable` doesn't ship a declarative remote API in nixpkgs. Either add the `flatpak-managed-install` overlay, write a one-shot systemd unit that runs `flatpak remote-add --if-not-exists flathub …`, or surface the manual step in `nomarchy-welcome`.
 - **First-run welcome wizard.** Extend `nomarchy-welcome` from a one-shot greeter into a guided picker: theme, panel position, monospace font, "what's a sane home.nix to start with?". Runs once, persists "done" in `state.json`.
 - **Plymouth theme variants per palette.** Currently one Plymouth theme; could template per-palette so the boot splash matches the active theme.
-- **`docs/TROUBLESHOOTING.md`.** The five most common rebuild errors (`option ... already declared`, `attribute ... missing`, Stylix target conflicts, home-manager backupFileExtension churn, impermanence path missing) with copy-paste fixes.
 - **Installer pre-flight resume polish.** If the user Ctrl-Cs mid-install and runs `--resume`, re-show the review screen before re-prompting for any unsaved fields.
 
 ### Later (speculative or research-shaped)
@@ -125,6 +124,7 @@ Each PR description should reference the row(s) in `docs/SCRIPTS.md` it closes,
 
 (Move items here when they land — keep them brief, link the commit/PR.)
 
+- _2026-04-26_ — `docs/TROUBLESHOOTING.md`. The five most common rebuild errors (option-already-declared, attribute-missing, Stylix target conflict, home-manager `.hm-bak` churn, impermanence path missing) with copy-paste fixes. Linked from `README.md` and `docs/MIGRATION.md`.
 - _2026-04-26_ — Gaming preset module (`core/system/gaming.nix`). Opt-in `nomarchy.system.gaming.enable` (default false). Wires `programs.steam` (with `remotePlay`/`localNetworkGameTransfers` firewall holes via `mkDefault`), `programs.gamemode`, and `services.flatpak`. Flathub remote and Hyprland window-rule split into separate Next-column rows.
 - _2026-04-26_ — Accessibility preset module (`core/system/accessibility.nix`). New `nomarchy.system.accessibility.{enable,cursorSize}` options (opt-in, default off — accessibility isn't a hardware-derived signal). Enables `services.gnome.at-spi2-core`, installs Orca, and sets `XCURSOR_SIZE=32` (configurable). Hyprland-side companion (key-repeat slowdown, Orca keybinding, high-contrast palette) split into a new Next-column row.
 - _2026-04-26_ — Desktop preset module (`core/system/desktop.nix`). New `nomarchy.system.desktop.enable` option; defaults to `formFactor == "desktop"` (mirror of the laptop preset's auto-enable). Pins `powerManagement.cpuFreqGovernor` to `"performance"` and enables `services.zfs.{autoScrub,trim}` so a future ZFS pool gets sensible maintenance for free.