core: split math module into focused submodules

.githooks/pre-commit

@@ -0,0 +1,53 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Enforce coupling between PRNG algorithm/version and golden regression vector.
+
+PRNG_FILE="crates/rmg-core/src/math/prng.rs"
+
+# Only run if the PRNG file is staged
+if ! git diff --cached --name-only | grep -qx "$PRNG_FILE"; then
+  exit 0
+fi
+
+DIFF=$(git diff --cached -- "$PRNG_FILE" || true)
+
+# Heuristics to detect algorithm changes: edits to these functions imply behavior change
+if echo "$DIFF" | grep -E '^(\+|-)\s*(fn\s+next_u64|fn\s+from_seed_u64|fn\s+from_seed\(|fn\s+next_int\()' >/dev/null; then
+  ALGO_CHANGED=1
+else
+  ALGO_CHANGED=0
+fi
+
+# Version bump present?
+if echo "$DIFF" | grep -E 'PRNG_ALGO_VERSION' >/dev/null; then
+  VERSION_CHANGED=1
+else
+  VERSION_CHANGED=0
+fi
+
+# Golden regression vector updated?
+if echo "$DIFF" | grep -E 'next_int_golden_regression|assert_eq!\(values,\s*vec!\[' >/dev/null; then
+  GOLDEN_CHANGED=1
+else
+  GOLDEN_CHANGED=0
+fi
+
+FAIL=0
+if [[ "$ALGO_CHANGED" -eq 1 && "$VERSION_CHANGED" -eq 0 ]]; then
+  echo "pre-commit: PRNG algorithm changed but PRNG_ALGO_VERSION was not bumped." >&2
+  FAIL=1
+fi
+
+if [[ "$VERSION_CHANGED" -eq 1 && "$GOLDEN_CHANGED" -eq 0 ]]; then
+  echo "pre-commit: PRNG_ALGO_VERSION bumped but golden regression vector was not updated." >&2
+  FAIL=1
+fi
+
+if [[ "$FAIL" -eq 1 ]]; then
+  echo "pre-commit: Refusing commit. Update algorithm version and golden regression together." >&2
+  exit 1
+fi
+
+exit 0
+

.github/workflows/ci.yml

@@ -52,6 +52,8 @@ jobs:
             .
       - name: cargo test
         run: cargo test
+      - name: PRNG golden regression (rmg-core)
+        run: cargo test -p rmg-core --features golden_prng -- tests::next_int_golden_regression
 
   docs:
     name: Docs Guard

Makefile

@@ -0,0 +1,8 @@
+SHELL := /bin/bash
+
+.PHONY: hooks
+hooks:
+	@git config core.hooksPath .githooks
+	@chmod +x .githooks/* 2>/dev/null || true
+	@echo "[hooks] Installed git hooks from .githooks (core.hooksPath)"
+

crates/rmg-cli/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "rmg-cli"
 version = "0.1.0"
-edition = "2024"
+edition = "2021"
 
 [dependencies]

crates/rmg-core/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "rmg-core"
 version = "0.1.0"
-edition = "2024"
+edition = "2021"
 
 [dependencies]
 blake3 = "1"
@@ -12,3 +12,9 @@ thiserror = "1"
 once_cell = "1"
 serde = { version = "1", features = ["derive"] }
 serde_json = "1"
+
+[features]
+default = []
+# Optional regression check for PRNG sequences; off by default to avoid
+# freezing algorithm choices. Used only in tests guarded with `cfg(feature)`.
+golden_prng = []

crates/rmg-core/src/lib.rs

@@ -411,12 +411,13 @@ fn add_vec(a: [f32; 3], b: [f32; 3]) -> [f32; 3] {
 
 /// Executor that updates the encoded position in the entity payload.
 fn motion_executor(store: &mut GraphStore, scope: &NodeId) {
-    if let Some(record) = store.node_mut(scope)
-        && let Some(payload) = &record.payload
-        && let Some((position, velocity)) = decode_motion_payload(payload)
-    {
-        let updated = encode_motion_payload(add_vec(position, velocity), velocity);
-        record.payload = Some(updated);
+    if let Some(record) = store.node_mut(scope) {
+        if let Some(payload) = &record.payload {
+            if let Some((position, velocity)) = decode_motion_payload(payload) {
+                let updated = encode_motion_payload(add_vec(position, velocity), velocity);
+                record.payload = Some(updated);
+            }
+        }
     }
 }