refactor: add main.ts decomposition guardrails and extract core helpers

src/core/utils/coerce.ts

@@ -0,0 +1,31 @@
+export function asFiniteNumber(
+  value: unknown,
+  fallback: number,
+  min?: number,
+  max?: number,
+): number {
+  if (typeof value !== "number" || !Number.isFinite(value)) return fallback;
+  if (min !== undefined && value < min) return min;
+  if (max !== undefined && value > max) return max;
+  return value;
+}
+
+export function asString(value: unknown, fallback: string): string {
+  if (typeof value !== "string") return fallback;
+  const trimmed = value.trim();
+  return trimmed.length > 0 ? trimmed : fallback;
+}
+
+export function asBoolean(value: unknown, fallback: boolean): boolean {
+  if (typeof value === "boolean") return value;
+  if (typeof value === "string") {
+    const normalized = value.trim().toLowerCase();
+    if (normalized === "yes" || normalized === "true" || normalized === "1") {
+      return true;
+    }
+    if (normalized === "no" || normalized === "false" || normalized === "0") {
+      return false;
+    }
+  }
+  return fallback;
+}