fix(simulation): address PR #122 review — determinism, correctness, labeling

- HashMap → BTreeMap throughout econ-sim for deterministic iteration (D-010)
- Fix cost_factor: multiplicative gate×zone instead of additive (trade.rs)
- Extract derive_seed to shared prng.rs, consolidate FNV-1a implementation
- Rename run_shock_test → run_no_explosion_check (not D-179 Test 3)
- Deduplicate cross-zone FX rate collection in Test 4
- Replace ORDER BY RANDOM() with deterministic ordering + ChaCha8Rng
- Make commodity coverage failure a hard error consistent with D-175
- Fix gap-fill off-by-one (4 corps → 3 when coverage = 0)
- Correct test report: EconEvent exists, location_type is body/station

All four D-179 stability tests still pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tooling/econ-sim/src/prng.rs

@@ -0,0 +1,35 @@
+//! Shared PRNG helpers for deterministic seeding (D-176, D-174).
+//!
+//! Both seed.rs and currency.rs use the same FNV-1a mix + Box-Muller transform.
+//! Centralised here to guarantee identical derivation chains across modules.
+
+use std::f64::consts::PI;
+
+use rand::Rng;
+use rand_chacha::ChaCha8Rng;
+
+/// FNV-1a 64-bit offset basis.
+const FNV_OFFSET_BASIS: u64 = 0xcbf29ce484222325;
+
+/// FNV-1a 64-bit prime.
+const FNV_PRIME: u64 = 0x100000001b3;
+
+/// Deterministic per-key seed: FNV-1a of `key` mixed with `run_seed`.
+///
+/// Starting from `run_seed + FNV_OFFSET_BASIS` provides per-run variation
+/// while preserving the FNV avalanche properties across keys.
+pub fn derive_seed(run_seed: u64, key: &str) -> u64 {
+    let mut h = run_seed.wrapping_add(FNV_OFFSET_BASIS);
+    for byte in key.bytes() {
+        h ^= byte as u64;
+        h = h.wrapping_mul(FNV_PRIME);
+    }
+    h
+}
+
+/// Box-Muller transform: standard normal variate from a ChaCha8 stream.
+pub fn standard_normal(rng: &mut ChaCha8Rng) -> f64 {
+    let u1: f64 = 1.0 - rng.random::<f64>(); // avoid ln(0)
+    let u2: f64 = rng.random::<f64>();
+    (-2.0 * u1.ln()).sqrt() * (2.0 * PI * u2).cos()
+}