refactor(simulation): split atlas.rs subcommands into modules

Extract 2119-line monolithic atlas.rs into 8 focused modules under
src/bin/atlas/: main.rs (thin dispatch), common.rs (shared types and
DB helpers), show.rs, mutate.rs, stats.rs, systems.rs, author.rs,
and sync_wiki.rs. Commands enum stays in main.rs; each match arm
delegates to module::cmd_fn(&conn, args). No behavior change.

Closes #776.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/Cargo.lock

@@ -1236,7 +1236,7 @@ dependencies = [
 
 [[package]]
 name = "settled-reach-server"
-version = "0.1.29"
+version = "0.1.30"
 dependencies = [
  "bevy_app",
  "bevy_ecs",