feat(simulation): T-1151 window par_iter + T-1150 granularity carrier (five touch points + aliasing tests)

T-1151: build_district_window_layer dispatches one Rayon task per row
(pure derive_window_cell via derive_at_metres), scattered row-major into
the flat arrays; a cfg(test) serial path backs the bit-identical
parallel-vs-serial golden.

T-1150: serde-default window_granularity (1=district, 4=quarter) +
window_min_wl_m on AtlasLayerRequest — additive, no sixth demux shape,
old frames decode unchanged (tested). Quarter mode = full
reclassification at 512m spacing over the same world rect ((4n)x(4n)
cells); WIRE_CAP_CELLS=4096 enforces n*granularity <= cap (quarter
clamps n to 16, the design doc's worked example). Granularity + min_wl
key ALL five touch points: DistrictWindowLayer echo, server FIFO-256
cache key (now a 5-tuple), per-connection coalescing key, client
request codec (omitted-at-default wire fields), client LRU key.

Mandatory aliasing regressions on both ends: identical (body, center, n)
at granularity 1 vs 4 produce distinct cache entries and correct
per-granularity payload shapes (server, 3-thread queue to avoid the
AnalyzeBody thread contention found while writing it) and distinct
client cache keys (gdUnit). Replay fixture regenerated — the layer
struct grew two echoed fields (231->254 bytes, content verified).

Client requests stay district-granularity by default — quarter requests
arrive with T-1153's rung selection.

server/src/atlas/plugin.rs

@@ -413,7 +413,10 @@ fn drain_generation_completions(
                 // NEXT poll (the existing D-225 re-request loop) hits
                 // `handle_atlas_request`'s window branch, which finds this
                 // entry via `DistrictWindowCache::get` and serves it.
-                window_cache.insert((body_id, layer.center, layer.n), *layer);
+                window_cache.insert(
+                    (body_id, layer.center, layer.n, layer.granularity, layer.min_wl_m),
+                    *layer,
+                );
             }
         }
     }
@@ -904,6 +907,8 @@ mod tests {
                 up_to: CascadeLayer::Topography,
                 window_center: None,
                 window_n: 0,
+                window_granularity: 0,
+                window_min_wl_m: 0,
             },
         )]));
         world.insert_resource(AtlasResponseBuffer::default());