Plugin system for engine modules

[PrograMistV1]

Context

LightExtract is currently hardcoded into ExtractSchedule::default()
(engine-core/src/render/extract/mod.rs), which means it runs even for
pipelines with no scene concept (e.g. LoadingPipeline). See the TODO in
engine-core/src/render/gfx/light_buffer.rs:

DirectionalLightComponent/PointLightComponent, LightExtract,
DirectionalLight/GpuPointLight/ExtractedLights are a concrete lighting
implementation for a single pipeline, not a general core plumbing
abstraction (see README: "engine-core has no opinion about how you
render things"). Move it to engine-pipelines once a plugin/extension
system for ExtractSchedule exists.

This is a general problem, not just a lighting one: any future module
(physics, audio, particles, ...) will hit the same wall - engine-core
would otherwise need a growing list of App::register_x() methods, one
per subsystem.

Goal

Design and implement a plugin system for engine-core, so that modules
(engine-pipelines and future crates) register what they need
(extract systems, tick systems, asset loaders, ...) through one
consistent mechanism, instead of engine-core hardcoding them into
ExtractSchedule::default() / TickSchedule::default() / etc.

Scope

• Add EnginePlugin trait to engine-core (fn build(&self, app: &mut EngineContext))
• Add builder methods on EngineContext: add_extract_system,
  add_tick_system, add_plugin, add_plugins
• Remove LightExtract from ExtractSchedule::default()
• Add LightingPlugin in engine-pipelines that registers LightExtract
• Update engine-demo (sponza_scene_demo.rs, interpolation_demo.rs)
  to call ctx.add_plugin(LightingPlugin) in on_start
• Resolve the light_buffer.rs TODO (remove/update the comment once done)
• Decide whether register_loaders (currently a separate App method)
  should be folded into the same plugin mechanism, or stay separate