docs(simulation): correct stale cascade docs and pin layer order (#955)

Address the doc/test-correctness items from PR #149 review:

- cascade.rs: Layer 3 is RNG-free (pure fn of attractors+cities); it does
  not consume the carried SeedChain. Corrected the "first RNG-using layer"
  claims on the module doc and the Settlement variant (Hoshe H3).
- cascade.rs: layers_are_ordered now also asserts Topography < Settlement,
  pinning the invariant the up_to >= Settlement guards rely on (Tyre T2).
- features.rs: strength is an integer 0-100 (quantized at extraction), not
  f32, and consumers that rank by it use the integer value (Hoshe H2).
- layer_proxy.rs: AtlasLayerRequest.up_to is not yet honored — run_work_item
  runs through Settlement unconditionally; per-request depth deferred to
  #1021 (Tyre T1).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/src/atlas/layer_proxy.rs

@@ -25,8 +25,12 @@ use crate::seed::SeedChain;
 /// (the loader prefers the chunk; this is only the floor).
 const DEFAULT_SEA_LEVEL: f32 = 0.3;
 
-/// A client request for a body's generation layers (D-225). `up_to` is a
-/// forward-compat seam — v1 always runs the Layer-1 (Topography) cascade.
+/// A client request for a body's generation layers (D-225).
+///
+/// `up_to` is a forward-compat seam that is **not yet honored**: `run_work_item`
+/// currently runs the cascade through `CascadeLayer::Settlement` unconditionally,
+/// ignoring this field. Wiring per-request depth (and the partial caching it
+/// implies) is deferred to #1021.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct AtlasLayerRequest {
     pub body_id: String,