Rework features into a Feature class with per field events

Review points from Cooldude:

- Feature.register(name[, config]) is required again and returns a
  feature object. Each feature has its config, its fields, and its
  overrides, so one registry holds all the data. Only the overrides are
  stored. On load each feature points at its own stored table again.
- The methods are feature:guard(lib), feature:update_config(enabled,
  values), and feature:is_enabled(). Feature.get(name) looks up a
  feature, and commands flag the feature object.
- update_config raises on_config_updated once per changed value, with
  the feature name, path, old value, and new value. The gui refresh only
  reacts to the "enabled" path.
- apply only sets values. on_apply is gone. Lookup sets built from
  lists use Feature.to_set, which is cached by the list itself. A
  replaced list gets a new set, so the sets are also correct after a
  load. Protection exposes getters for its protected names and types.

Web: settings use small inputs, the same size as the config pages.
Fields are shaped like clusterio config field definitions. A field
with an inputComponent renders the component registered with the web
interface, so the role setting now uses core's role input, and other
plugin inputs can be used later.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

exp_scenario/module/control/spawn_area.lua

@@ -2,8 +2,9 @@
 Adds a custom spawn area with chests and afk turrets
 ]]
 
-local Features = require("modules/exp_scenario/features")
-local config = require("modules/exp_scenario/config/spawn_area")
+local Feature = require("modules/exp_scenario/features")
+local feature = require("modules/exp_scenario/config/spawn_area")
+local config = feature.config
 
 --- Apply an offset to a LuaPosition
 --- @param position MapPosition.struct
@@ -299,9 +300,9 @@ end
 
 local e = defines.events
 
-return Features.guard(config, {
+return feature:guard{
     on_nth_tick = on_nth_tick,
     events = {
         [e.on_player_created] = on_player_created,
     },
-})
+}