wal, block, sse, zephyr: remove [mutable] from never-reassigned fields

Warning 69 (unused-field, mutable-never-assigned). Four independent
record fields were flagged as mutable but the code only mutates their
referents in place, never rebinds the record slot itself:

- ocaml-wal/lib/wal.ml: [t.file] (the Eio file resource; methods call
Eio.File.pwrite_all etc., the slot is set once at open time).
- ocaml-block/lib/block.ml: [Memory.state.data] (the backing bytes,
written via Bytes.blit_string; [Bytes.t] is already mutable).
- ocaml-sse/lib/sse.ml: [Parser.t.data_buf] (a Buffer.t, written via
Buffer.add_*; the slot never changes).
- ocaml-zephyr/lib/zephyr.ml: drop [mode : Read | Write] entirely —
set at open-time, read nowhere. The open_read / open_write
constructors already distinguish the two call shapes, so mode
tracking was redundant.