feat(base-formula): add string and coercion functions

packages/base-formula/src/functions/coercion.ts

@@ -0,0 +1,17 @@
+// packages/base-formula/src/functions/coercion.ts
+import { register } from "./registry";
+
+register({
+  name: "toNumber", arity: { min: 1, max: 1 }, paramTypes: "any", returnType: "number",
+  eval: ([v]) => {
+    if (v == null) return null;
+    const n = Number(v);
+    return Number.isFinite(n) ? n : null;
+  },
+  doc: "Parses the value as a number, or null.", category: "coercion",
+});
+register({
+  name: "toString", arity: { min: 1, max: 1 }, paramTypes: "any", returnType: "string",
+  eval: ([v]) => v == null ? "" : String(v),
+  doc: "Converts the value to a string.", category: "coercion",
+});

packages/base-formula/src/functions/string.ts

@@ -0,0 +1,35 @@
+// packages/base-formula/src/functions/string.ts
+import { register } from "./registry";
+
+const s = (v: unknown): string => v == null ? "" : String(v);
+
+register({
+  name: "concat", arity: { min: 1, max: null }, paramTypes: "variadic-any", returnType: "string",
+  eval: (args) => args.map(s).join(""),
+  doc: "Concatenates strings.", category: "string",
+});
+register({
+  name: "length", arity: { min: 1, max: 1 }, paramTypes: ["string"], returnType: "number",
+  eval: ([v]) => s(v).length,
+  doc: "Length of a string.", category: "string",
+});
+register({
+  name: "contains", arity: { min: 2, max: 2 }, paramTypes: ["string", "string"], returnType: "boolean",
+  eval: ([a, b]) => s(a).includes(s(b)),
+  doc: "Returns true if the first string contains the second.", category: "string",
+});
+register({
+  name: "lower", arity: { min: 1, max: 1 }, paramTypes: ["string"], returnType: "string",
+  eval: ([v]) => s(v).toLowerCase(),
+  doc: "Lowercases the string.", category: "string",
+});
+register({
+  name: "upper", arity: { min: 1, max: 1 }, paramTypes: ["string"], returnType: "string",
+  eval: ([v]) => s(v).toUpperCase(),
+  doc: "Uppercases the string.", category: "string",
+});
+register({
+  name: "trim", arity: { min: 1, max: 1 }, paramTypes: ["string"], returnType: "string",
+  eval: ([v]) => s(v).trim(),
+  doc: "Strips whitespace from both ends.", category: "string",
+});