feat(state): validation + friendly errors on both write and eval paths (item 11)

'Never has to master Nix' includes the error messages. The tool grows
validate_state (appearance schema = hard errors; settings.* and
unknown keys = warnings so menu writers and newer schemas keep
working), a read-only `validate` subcommand, and validation BEFORE
every write — an invalid set/apply is refused with the on-disk file
untouched. JSON syntax errors now say line, column, and fix. theme.nix
enforces the same schema at eval time (on the post-defaults state, so
sparse/older files still evaluate) and throws a message naming each
field, its got-value, and the fix paths — the hand-edit escape hatch
no longer ends in a Nix stack trace. Doctor's JSON fix-text points at
validate.

V1: new checks.theme-sync-validate fixture corpus green (good state,
trailing comma, bad hex, bad type, unknown key, refused set with
byte-identical file); negative mkFlake eval of a broken scratch
downstream shows the friendly throw; good-path template-home builds.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkgs/nomarchy-doctor/nomarchy-doctor.sh

@@ -72,7 +72,7 @@ if [ -f "$flake/theme-state.json" ]; then
     ok "theme-state.json parses"
   else
     bad "theme-state.json is not valid JSON (rebuilds will fail)" \
-        "jq . $flake/theme-state.json  (shows the bad spot; fix it, or re-apply a theme)"
+        "nomarchy-theme-sync validate  (names the spot; fix it, or re-apply a theme)"
   fi
   if git -C "$flake" ls-files --error-unmatch theme-state.json >/dev/null 2>&1; then
     ok "theme-state.json is git-tracked"