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/env.el

@@ -0,0 +1,77 @@
+// runtime/env.el — environment and process
+// Covers: environment variables, command-line args, process exit, in-process
+// state store, UUID generation, and list convenience helpers.
+
+// env — read an environment variable.  Returns "" if the variable is not set.
+fn env(key: String) -> String {
+    return __env_get(key)
+}
+
+// args — command-line arguments as a list of strings.
+// __args_json returns a JSON array (e.g. ["prog","arg1","arg2"]).
+// The list is built by iterating over the array.
+fn args() -> [String] {
+    let json: String = __args_json()
+    let n: Int = json_array_len(json)
+    let result: [String] = el_list_empty()
+    let i: Int = 0
+    while i < n {
+        let item: String = json_array_get_string(json, i)
+        let result = el_list_append(result, item)
+        let i = i + 1
+    }
+    return result
+}
+
+// exit_program — terminate the process with the given exit code.
+fn exit_program(code: Int) {
+    __exit_program(code)
+}
+
+// ── List convenience helpers ───────────────────────────────────────────────
+
+// get — index into a list.  Thin alias for el_list_get used throughout the
+// El stdlib so call sites read like `get(lst, i)` rather than the verbose form.
+fn get(lst: [String], i: Int) -> String {
+    return el_list_get(lst, i)
+}
+
+// len — length of a list.
+fn len(lst: [String]) -> Int {
+    return el_list_len(lst)
+}
+
+// ── In-process key-value state store ──────────────────────────────────────
+
+// state_set — store a string value under key.
+fn state_set(key: String, val: String) {
+    __state_set(key, val)
+}
+
+// state_get — retrieve value for key; returns "" if key not present.
+fn state_get(key: String) -> String {
+    return __state_get(key)
+}
+
+// state_del — remove key from the store.
+fn state_del(key: String) {
+    __state_del(key)
+}
+
+// state_keys — all keys currently in the store as a JSON array string.
+fn state_keys() -> String {
+    return __state_keys()
+}
+
+// ── UUID generation ────────────────────────────────────────────────────────
+
+// uuid_new — generate a new random UUID v4.
+fn uuid_new() -> String {
+    return __uuid_v4()
+}
+
+// uuid_v4 — alias for uuid_new(); explicit version name for callers that
+// need to be precise about the UUID variant.
+fn uuid_v4() -> String {
+    return __uuid_v4()
+}