relayburn-sdk: drop reintroduced let _ block in parse_codex_buffer (#405)

* relayburn-sdk: drop reintroduced let _ block in parse_codex_buffer

PR #371 dropped the two `let _ = (...)` warning-silencers at the tail of
`parse_codex_buffer`, but #372 (JSONL streaming refactor) re-added the
larger one when it restructured the loop. Audit re-confirms each
`committed_*` mirror is read either by the emitted records or by the
resume state, so the live values are dead at function exit and modern
rustc does not flag them.

Refs #346.

* changelog: trim parse_codex_buffer entry to impact-first style

Per project changelog guidelines and CodeRabbit feedback on #405.

* changelog: drop noop dead-code removal entry

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: willwashburn

crates/relayburn-sdk/src/reader/codex.rs

@@ -1192,18 +1192,6 @@ fn parse_codex_buffer<R: BufRead>(
         }
     }
 
-    // Silence unused-mutable warnings for snapshot mirrors that are written
-    // but only read indirectly.
-    let _ = (
-        cumulative,
-        session_id,
-        session_cwd,
-        turn_contexts,
-        seen_session_meta_keys,
-        root_session_emitted,
-        last_completed_turn,
-    );
-
     Ok(ParseCodexIncrementalResult {
         turns,
         content: content_out,