feat(alias): filename -> snake_case alias converter

src/alias.js

@@ -0,0 +1,30 @@
+// src/alias.js
+// Pure functions for deriving snake_case aliases from sprite filenames.
+// No I/O.
+
+const path = require('node:path');
+const SUPPORTED_EXT = ['.png', '.webp', '.jpg', '.jpeg'];
+
+function stripPrefix(name, prefix) {
+    if (!prefix) return name;
+    if (name.startsWith(prefix)) return name.slice(prefix.length);
+    return name;
+}
+
+// CamelCase -> snake_case. Boundary: lowercase/digit -> uppercase.
+function camelToSnakeCase(s) {
+    return s
+        .replace(/([a-z0-9])([A-Z])/g, '$1_$2')
+        .toLowerCase();
+}
+
+function aliasFromFilename(filename, opts = {}) {
+    const ext = path.extname(filename).toLowerCase();
+    let base = SUPPORTED_EXT.includes(ext)
+        ? filename.slice(0, -ext.length)
+        : filename;
+    if (opts.stripPrefix) base = stripPrefix(base, opts.stripPrefix);
+    return camelToSnakeCase(base);
+}
+
+module.exports = { stripPrefix, camelToSnakeCase, aliasFromFilename };