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/cascade.rs

@@ -11,9 +11,10 @@
 //! path-loading [`run_cascade`] is a thin wrapper around it.
 //!
 //! **Determinism (D-010 #4):** for a fixed heightmap + [`SeedChain`], the
-//! snapshot is reproducible. Layers 0–1 are RNG-free (pure functions of the
+//! snapshot is reproducible. Layers 0–3 are RNG-free — Layers 0–1 are pure
-//! heightmap); the carried `SeedChain` feeds the RNG-using layers (Layer 3+,
+//! functions of the heightmap, and Layer 3 is a pure function of
-//! D-224).
+//! (attractors, cities). The carried `SeedChain` is reserved for the future
+//! RNG-using layers (Layer 4+, D-224).
 
 use std::path::Path;
 
@@ -36,7 +37,9 @@ pub enum CascadeLayer {
     /// Layer 1 — empty-world topography: drainage, feature tags, sub-biome (#953).
     Topography,
     /// Layer 3 — settlement placement: attractor-matched city positions (#955, D-211).
-    /// First RNG-using layer (uses the carried `SeedChain`).
+    /// Deterministic and RNG-free: a pure function of (attractors, cities) via
+    /// `match_cities`. The carried `SeedChain` is unused here; later stochastic
+    /// layers (Layer 4+) will consume it.
     Settlement,
 }
 
@@ -258,7 +261,12 @@ mod tests {
 
     #[test]
     fn layers_are_ordered() {
+        // The `up_to >= CascadeLayer::Settlement` guards in the cascade rely on
+        // this declaration order — pin it explicitly so reordering the enum (or
+        // inserting a layer out of sequence) fails here instead of silently
+        // breaking which layers run.
         assert!(CascadeLayer::Heightmap < CascadeLayer::Topography);
+        assert!(CascadeLayer::Topography < CascadeLayer::Settlement);
     }
 
     #[test]

server/src/atlas/features.rs

@@ -2,8 +2,10 @@
 //!
 //! After D8 drainage analysis (D-208), this module extracts the 7
 //! `AttractorType` tags from the heightmap + river network. Each attractor has
-//! a pixel position and a normalized `strength` (0.0–1.0) derived from local
+//! a pixel position and a `strength` (integer 0–100) derived from local terrain
-//! terrain quality.
+//! quality — computed in floating point, then quantized to an integer at the
+//! extraction boundary so all downstream decisions stay integer-deterministic
+//! (D-010).
 //!
 //! **D-209 / D-223 reconciliation:** D-209 reads ocean/lake polygons from
 //! `markers.json`, but D-223 reduced markers to a names-only pool — those
@@ -13,8 +15,10 @@
 //!
 //! **Determinism (D-010 #4):** all collections iterate in sorted/row-major
 //! order; the final attractor list is sorted by `(attractor_type, row, col)`.
-//! No `HashMap`/`HashSet` iteration. `strength` is f32 but is never used as a
+//! No `HashMap`/`HashSet` iteration. `strength` (integer 0–100) is not part of
-//! sort key.
+//! that ordering here, so float quantization can't perturb the sort; consumers
+//! that rank by strength (e.g. `layer1::attach_feature_names`) do so on the
+//! integer value.
 
 use std::collections::{BTreeMap, VecDeque};
 

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
+/// A client request for a body's generation layers (D-225).
-/// forward-compat seam — v1 always runs the Layer-1 (Topography) cascade.
+///
+/// `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,