fix(simulation): prosperity in basis points + shared log10_floor helper (#954)

Address PR #145 review. Convert prosperity from f32 to integer basis
points (matching the existing prosperity_bps, D-010 integer-only),
removing both float-determinism bugs by construction:

- New server/src/bps.rs: log10_floor (integer order-of-magnitude) +
  bps_to_f32 edge helper, with boundary tests.
- city_context_reader: prosperity_baseline_bps (u32, 0-10000). role base
  + pop bonus (400*log10_floor, cap 1200) + symmetric noise via integer
  modulo (fixes the always-negative bug) + clamp [1000,9500]. No floats.
- CityGenerationContext.prosperity_baseline -> prosperity_baseline_bps;
  updated the two test context builders. Not serialized — no wire break.
- district_mix population_tier now uses log10_floor (same determinism
  bug class as the comment claimed to avoid).
- Tests in bps + assert positive noise is achievable (the case the old
  test hid).

cargo check/clippy --all-targets -D warnings clean; 1291 lib tests pass;
fmt clean.

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

server/src/atlas/gen_queue.rs

@@ -468,7 +468,7 @@ mod tests {
             context: Box::new(CityGenerationContext {
                 city_id,
                 political_archetype: PoliticalArchetype::Commission,
-                prosperity_baseline: 0.6,
+                prosperity_baseline_bps: 6_000,
                 surrounding_biome: SettingType::Urban,
                 road_entry_directions: vec![],
                 footprint_radius_km: 5.0,