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/exec.el

@@ -0,0 +1,27 @@
+// runtime/exec.el — subprocess execution
+// All four names resolve to the same seed primitives so callers can use
+// whichever name matches their mental model of the operation.
+
+// exec — run a shell command, capture stdout, return as String.
+// Blocks until the subprocess exits (30-second wall-clock deadline in the
+// seed layer).  Returns "" on any error.
+fn exec(cmd: String) -> String {
+    return __exec(cmd)
+}
+
+// exec_bg — fire-and-forget subprocess.  Returns immediately; no stdout.
+fn exec_bg(cmd: String) {
+    __exec_bg(cmd)
+}
+
+// exec_command — alias for exec(); preferred when callers care about side
+// effects (e.g. invoking a build tool) rather than captured output.
+fn exec_command(cmd: String) -> String {
+    return __exec(cmd)
+}
+
+// exec_capture — alias for exec(); preferred when callers explicitly want
+// to capture and process stdout.
+fn exec_capture(cmd: String) -> String {
+    return __exec(cmd)
+}