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.

lib/state-schema.nix

@@ -1,5 +1,13 @@
 # Nomarchy State Schema
-# Defines the complete state shape with defaults for both home and system state
+#
+# Defines the default values for every state.json field that's consumed by a
+# Nix option. Read by core/{system,home}/options.nix (for `default = …`) and
+# by core/{system,home}/state.nix (for `or` fallbacks).
+#
+# state.json may also hold runtime-only fields that aren't declared here —
+# notably `welcome_done`, managed by `nomarchy-welcome`. Those are intentionally
+# off-schema because no Nix option reads them; the schema is the "consumed by
+# Nix" surface, not the full state.json shape.
 { lib }:
 
 {
@@ -54,14 +62,4 @@
     };
   };
 
-  # Get a value from state with fallback to default
-  getWithDefault = state: path: default:
-    let
-      pathList = lib.splitString "." path;
-      getValue = obj: remaining:
-        if remaining == [] then obj
-        else if builtins.isAttrs obj && builtins.hasAttr (builtins.head remaining) obj
-        then getValue obj.${builtins.head remaining} (builtins.tail remaining)
-        else default;
-    in getValue state pathList;
 }