add runtime/fs.el, exec.el, env.el — filesystem, subprocess, environment in El

Migrates fs_read/write/exists/mkdir/write_bytes/list, exec/exec_bg/exec_command/exec_capture,
env/args/exit_program, state_set/get/del/keys, uuid_new/v4, and list helpers get/len from
el_runtime.c into El source as thin wrappers over seed primitives.

runtime/fs.el

@@ -0,0 +1,49 @@
+// runtime/fs.el — filesystem operations
+// Thin El wrappers over seed primitives; no logic beyond what is needed
+// to present a clean API.  The heavy lifting lives in el_runtime.c.
+
+fn fs_read(path: String) -> String {
+    return __fs_read(path)
+}
+
+fn fs_write(path: String, content: String) -> Bool {
+    return __fs_write(path, content)
+}
+
+fn fs_exists(path: String) -> Bool {
+    return __fs_exists(path)
+}
+
+fn fs_mkdir(path: String) -> Bool {
+    return __fs_mkdir(path)
+}
+
+fn fs_write_bytes(path: String, bytes: String, n: Int) -> Bool {
+    return __fs_write_bytes(path, bytes, n)
+}
+
+// fs_list — return list of filenames in a directory.
+// __fs_list_raw returns a newline-separated string (possibly with a trailing
+// newline); callers that need a clean list should filter empty strings.
+fn fs_list(path: String) -> [String] {
+    let raw: String = __fs_list_raw(path)
+    return str_split(raw, "\n")
+}
+
+// fs_list_json — return a JSON array of filenames in a directory.
+// Empty strings produced by a trailing newline are stripped before encoding.
+fn fs_list_json(path: String) -> String {
+    let items: [String] = fs_list(path)
+    let n: Int = el_list_len(items)
+    let clean: [String] = el_list_empty()
+    let i: Int = 0
+    while i < n {
+        let item: String = el_list_get(items, i)
+        let trimmed: String = str_trim(item)
+        if !str_eq(trimmed, "") {
+            let clean = el_list_append(clean, "\"" + trimmed + "\"")
+        }
+        let i = i + 1
+    }
+    return json_build_array(clean)
+}