feat(simulation): T-1152 server half — WindowGranularity enum, derive_orbital_at_metres, Region rung on the same carrier

R1 measured first: a capped Region tile through the real production path
(build_district_window_layer + T-1151 par_iter) at the 64x64 wire cap
costs 0.40-0.48ms — faster than the shipped district n=64 window, so
Jeroen's progressive capped-density tiling ruling is comfortably
interactive on-demand. Raw orbital derive ~0.9µs/cell (~2.3x faster than
full derive; region_baseline dominates, not invent_primitives).

R5 redesign: WindowGranularity enum (Quarter/District/Region), serde
named-variant per the RoadNodeKind precedent, spacing from D-243 scale::
constants — the single source of truth. Additive serde-default
window_granularity_v2 request field (None = legacy u32 path; v2 wins when
Some); DistrictWindowLayer.granularity_v2 always echoed. Legacy u32 echo
for Region uses reserved WINDOW_GRANULARITY_REGION_KEY = u32::MAX (never
a legal input) so the old slot cannot lie about aliasing. Cache and
coalescing keys carry the enum itself (Ord by declaration order, D-010).

n stays district-extent at every rung; Region's cell grid is a DIVISION
(round(n/100), min 1) with its own per-axis ceiling
DISTRICT_WINDOW_MAX_N_REGION=6400 and a bounded halving-loop clamp (no
closed form under the rounding division — the client mirror must
replicate the loop).

derive_orbital_at_metres: bilinear envelope reads + region_baseline
temperature, NO invent_primitives (proven by test — slope_q pinned 0),
routed through the shared build_district_profile classification tail so
the existing colorizer family renders orbital cells unchanged. R2
stepped-categorical behavior documented at the function, not implied.

Region aliasing + clamp/echo tests mirror the T-1150 discipline. 1813
lib tests green; clippy clean; fixture regenerated (254->278 bytes, new
echoed field).

server/src/atlas/gen_queue.rs

@@ -34,7 +34,9 @@ use crate::atlas::cascade::{run_cascade_from_heightmap, CascadeLayer};
 use crate::atlas::district_profile::{BodyParams, ClimateConstants, DistrictPos};
 use crate::atlas::features::TerrainAnalysis;
 use crate::atlas::heightmap::{load_heightmap_png, GRID_H, GRID_W};
-use crate::atlas::layer_proxy::{build_district_window_layer, DistrictWindowLayer};
+use crate::atlas::layer_proxy::{
+    build_district_window_layer, DistrictWindowLayer, WindowGranularity,
+};
 use crate::atlas::shell::{fill_chunk, FilledChunk};
 use crate::atlas::skeleton_gen::{assign_all_block_tags, generate_quarter_skeleton};
 use crate::atlas::trait_catalog_reader::ExteriorCatalog;
@@ -208,11 +210,18 @@ pub enum GenWorkItem {
         /// before this item is built — never trusted from the wire again here.
         center: DistrictPos,
         n: u32,
-        /// Derivation granularity (T-1150) — `WINDOW_GRANULARITY_DISTRICT` (1)
-        /// or `WINDOW_GRANULARITY_QUARTER` (4). Already resolved via
-        /// `resolve_window_granularity` by the caller.
-        granularity: u32,
+        /// Derivation granularity (T-1150, widened T-1152 to the full
+        /// [`WindowGranularity`] vocabulary — `District`/`Quarter` (finer)
+        /// plus `Region` (coarser, T-1152)). Already resolved via
+        /// `resolve_window_granularity_v2` by the caller — this is a
+        /// concrete rung, never a raw wire value.
+        granularity: WindowGranularity,
         /// Octave cutoff in whole metres (T-1149/T-1150), `0` = no cutoff.
+        /// Meaningless for `granularity: Region` (`derive_orbital_at_metres`
+        /// never calls the octave-scatter path this cuts off) but still
+        /// carried and echoed uniformly — see `derive_orbital_at_metres`'s
+        /// doc for why the field is harmless-but-unused there, not
+        /// special-cased away.
         min_wl_m: u32,
     },
 }
@@ -227,13 +236,17 @@ impl GenWorkItem {
 
     /// Coalescing key for `DeriveWindow` items only — `(connection, body,
     /// granularity)` (T-1150, design doc §3 [SOFT] recommendation, extending
-    /// T-1137's `(connection, body)`). `granularity` is part of the key so an
-    /// in-flight district-spacing (granularity 1) pan-burst is never
-    /// superseded by an unrelated quarter-spacing (granularity 4) request for
-    /// the same connection+body, and vice versa — the two rungs are separate
-    /// in-flight derives, not competing updates to the same one.
+    /// T-1137's `(connection, body)`; widened T-1152 to carry the full
+    /// [`WindowGranularity`] enum rather than the legacy `u32`, so `Region`
+    /// occupies its own coalescing slot exactly like `District`/`Quarter` do
+    /// — this is one of the five T-1150 touch points the R5 redesign must
+    /// carry the new representation through). `granularity` is part of the
+    /// key so an in-flight district-spacing pan-burst is never superseded by
+    /// an unrelated quarter- or region-spacing request for the same
+    /// connection+body, and vice versa — every rung is a separate in-flight
+    /// derive, not a competing update to the same one.
     /// `None` for every other variant (they don't coalesce this way).
-    pub fn window_supersede_key(&self) -> Option<(ConnectionId, &str, u32)> {
+    pub fn window_supersede_key(&self) -> Option<(ConnectionId, &str, WindowGranularity)> {
         if let GenWorkItem::DeriveWindow {
             body_id,
             conn_id,
@@ -1150,19 +1163,14 @@ mod tests {
     /// coalescing tests can exercise the granularity axis of
     /// `window_supersede_key()` without a second near-duplicate helper.
     fn derive_window(body_id: &str, conn_id: ConnectionId, center: DistrictPos) -> GenWorkItem {
-        derive_window_at(
-            body_id,
-            conn_id,
-            center,
-            crate::atlas::layer_proxy::WINDOW_GRANULARITY_DISTRICT,
-        )
+        derive_window_at(body_id, conn_id, center, WindowGranularity::District)
     }
 
     fn derive_window_at(
         body_id: &str,
         conn_id: ConnectionId,
         center: DistrictPos,
-        granularity: u32,
+        granularity: WindowGranularity,
     ) -> GenWorkItem {
         GenWorkItem::DeriveWindow {
             body_id: body_id.to_string(),
@@ -1316,21 +1324,11 @@ mod tests {
 
         let conn = ConnectionId(9);
         q.submit_window(
-            derive_window_at(
-                "GranBody",
-                conn,
-                (0, 0),
-                crate::atlas::layer_proxy::WINDOW_GRANULARITY_DISTRICT,
-            ),
+            derive_window_at("GranBody", conn, (0, 0), WindowGranularity::District),
             GenPriority::Immediate,
         );
         q.submit_window(
-            derive_window_at(
-                "GranBody",
-                conn,
-                (0, 0),
-                crate::atlas::layer_proxy::WINDOW_GRANULARITY_QUARTER,
-            ),
+            derive_window_at("GranBody", conn, (0, 0), WindowGranularity::Quarter),
             GenPriority::Immediate,
         );
         assert_eq!(
@@ -1352,21 +1350,11 @@ mod tests {
 
         let conn = ConnectionId(11);
         q.submit_window(
-            derive_window_at(
-                "SameGranBody",
-                conn,
-                (0, 0),
-                crate::atlas::layer_proxy::WINDOW_GRANULARITY_QUARTER,
-            ),
+            derive_window_at("SameGranBody", conn, (0, 0), WindowGranularity::Quarter),
             GenPriority::Immediate,
         );
         q.submit_window(
-            derive_window_at(
-                "SameGranBody",
-                conn,
-                (5, 5),
-                crate::atlas::layer_proxy::WINDOW_GRANULARITY_QUARTER,
-            ),
+            derive_window_at("SameGranBody", conn, (5, 5), WindowGranularity::Quarter),
             GenPriority::Immediate,
         );
         assert_eq!(