chore(lib): drop dead helpers, document schema boundary

Two unused helpers and a missing comment in the lib/ surface, found
during the Pillar 8 sweep.

- `readState` in `lib/default.nix` was exported but has no external
  callers — only `readHomeState` and `readSystemState` use it
  internally. Removed from the export list; the function stays in the
  let-block (still wraps the two public readers).

- `getWithDefault` in `lib/state-schema.nix` was a complete dead
  function: declared as a path-walking fallback helper but never called
  anywhere in the tree. core/{system,home}/state.nix use inline
  `togglesState.<key> or schema.<scope>.<key>` instead. Removed.

- Added a header comment to `lib/state-schema.nix` explaining the
  schema's boundary — it lists every state.json field consumed by a
  Nix option, but state.json may also hold runtime-only fields
  (`welcome_done` from `nomarchy-welcome`) that are intentionally
  off-schema because no Nix option reads them. Future readers will
  otherwise think welcome_done is an orphan.

Logged a Later-column roadmap row for consolidating `flake.nix`'s
palette/themeNames re-imports with `nomarchyLib` so the theme list has
one source of truth instead of two.

docs/ROADMAP.md

@@ -39,6 +39,7 @@ Guardrails (apply when adding anything):
 - **Forgejo release pipeline.** `vYY.MM.x` tags matching the upstream NixOS channel; the pipeline pushes the three ISOs and an updated `flake.lock` snapshot.
 - **Optional `nomarchy-installer-vm`** rebuilt as a real flake app (not a one-off shell script) so users can install Nomarchy into a libvirt VM declaratively.
 - **Surface support module** via the relevant `nixos-hardware` profile + Surface kernel patches behind a `nomarchy.hardware.isSurface` toggle.
+- **Consolidate palette imports in `flake.nix` via `nomarchyLib`.** `flake.nix:79-80` re-imports `./themes/palettes` and recomputes `themeNames` even though `lib/default.nix` already exports both. Two computations, same result today — drift risk tomorrow. Import `nomarchyLib = import ./lib { inherit lib; }` once and use `nomarchyLib.{palettes,themeNames}` to make `lib/default.nix` the single source of truth for the theme list.
 
 ## 3. Pillar: Script & menu audit