feat(simulation): add economics simulation binary with Layer 1+2 (#806, #807)

Adds tooling/econ-sim — a standalone Rust binary for the Phase 2 economics
simulation:

Layer 1 (Leontief production, #806):
- Deterministic per-run PRNG seeding of corp×site productivity (D-176)
- Fixed-coefficient production chains; scarcity cascades downstream (D-178)
- Per-capita population demand for finals and services
- Gate-energy demand reduction for fusion_fuel at connected nodes (D-186)
- Price adjustment via local tâtonnement

Layer 2 (spatial price equilibrium, #807):
- Damped tâtonnement trade flows along gate links (α=0.03, β=0.4, D-178)
- 8% transport cost per hop damps long-distance arbitrage
- Flows computed from pre-step snapshot; applied atomically
- --stability-check implements D-179 Tests 1 and 2:
  · Test 1: cold-start convergence ±5% at tick 100 → PASS (max 1.05%)
  · Test 2: long-run stability ±2% over ticks 900–999 → PASS (max 0.00%)

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

tooling/econ-sim/src/output.rs

@@ -0,0 +1,41 @@
+//! CSV output for simulation snapshots.
+
+use std::fs::File;
+use std::io::{self, BufWriter, Write};
+use std::path::Path;
+
+use crate::model::TickRecord;
+
+/// Write records to CSV. If `path` is None, writes to stdout.
+pub fn write_csv(records: &[TickRecord], path: Option<&Path>) -> io::Result<()> {
+    let header = "node_id,commodity_id,supply,demand,price,tick\n";
+
+    match path {
+        Some(p) => {
+            let file = File::create(p)?;
+            let mut w = BufWriter::new(file);
+            write!(w, "{}", header)?;
+            for r in records {
+                writeln!(
+                    w,
+                    "{},{},{:.4},{:.4},{:.4},{}",
+                    r.node_id, r.commodity_id, r.supply, r.demand, r.price, r.tick
+                )?;
+            }
+            w.flush()
+        }
+        None => {
+            let stdout = io::stdout();
+            let mut w = BufWriter::new(stdout.lock());
+            write!(w, "{}", header)?;
+            for r in records {
+                writeln!(
+                    w,
+                    "{},{},{:.4},{:.4},{:.4},{}",
+                    r.node_id, r.commodity_id, r.supply, r.demand, r.price, r.tick
+                )?;
+            }
+            w.flush()
+        }
+    }
+}