stdlib pages do not mark which functions are effect fn

[O6lvl4]

Found in a per-page audit where every documented function was executed.

io, fs, process, env, http and random pages list functions without saying which are effect fn. Calling one from a pure fn is a compile error (E006), so this is the first thing a reader needs and the pages are silent about it.

The split is not guessable, which is what makes it worth documenting:

• fs: everything is effect fn except temp_dir. The trap is exists / is_dir / is_file / is_symlink — they return Bool, so they read as pure, but they are effectful.
• io: read_line, print, read_all are effect; read_byte, read_n_bytes, write, write_bytes are pure.
• process: effect — exec, exit, stdin_lines, exec_in, exec_with_stdin, exec_status, spawn, kill. Pure — args, env, pid, sleep, is_alive.
• env: all effect except os.
• http: effect — serve, get, post, put, patch, delete, request. Everything else (response, json, with_headers, status, body, req_*, query_params, …) is pure.
• random: all four functions are effect fn and the page says nothing, so every example on it only works inside an effect fn.

stdlib/args.md already states this explicitly and can serve as the pattern.

Related: a false blanket claim

stdlib/overview.md line 99 says "All I/O functions are effect fn and return Result." Both halves are false — see the pure functions listed above, and fs.exists / is_dir / is_file / is_symlink are effect but return Bool, not Result. Readers will lean on that sentence precisely because the per-page marking is missing.