refactor(core, protocol): extract memory diff helper and add clarity comments

[kacy]

summary

this pass targets two areas flagged in a readability review of emberkv-core and ember-protocol.

ConcurrentKeyspace — memory diff helper

the signed-diff atomic update pattern appeared four times in set, incr_by, incr_by_float, and append:

let diff = new_size as isize - old_size as isize;
if diff > 0 {
    self.memory_used.fetch_add(diff as usize, ...);
} else if diff < 0 {
    self.memory_used.fetch_sub((-diff) as usize, ...);
}

extracted into fn adjust_memory(&self, old_size: usize, new_size: usize). no logic change — same atomics, same ordering, same behavior on diff == 0 (no-op).

keyspace.rs — clone comment

get() returns Ok(Some(e.value.clone())) where the value is Value::String(Bytes). added a brief inline note that this is a cheap refcount increment so readers don't mistake it for a deep copy.

command.rs — command_name() comment

the doc comment now explains why the match is explicit rather than derive-generated: a proc macro would obscure the string mappings, which are the thing most worth seeing at a glance when auditing command names.

what was tested

• cargo build -p emberkv-core -p ember-protocol — clean
• cargo test -p emberkv-core -p ember-protocol — 330 tests pass, 0 failures
• cargo fmt --check -p emberkv-core -p ember-protocol — no formatting issues