Arc-ify stored log text to remove per-line double clones (ticket 100)

DataValue::LogLiteral and IngestedLine now carry an Arc<str> instead
of an owned String, so the two clones the ingest path paid per line
(once to move text into the write closure, once to store it) become
cheap refcount bumps instead of full string copies. Also cuts the
per-string-content cost of left_right's full-EngineSnapshot deep clone
on each reconciliation sync.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

src/engine/snapshot/mod.rs

@@ -249,7 +249,7 @@ impl EngineSnapshot {
                         timestamp,
                     },
                     DataValue::LogLiteral { text },
-                ) => Some((*sequence, *timestamp, text.as_str())),
+                ) => Some((*sequence, *timestamp, text.as_ref())),
                 _ => None,
             })
     }

src/engine/snapshot/query.rs

@@ -106,7 +106,7 @@ fn logs_sorted_by_param(
         entries.push(LogEntryDto {
             sequence,
             timestamp_ms: timestamp.as_millis(),
-            text: text.clone(),
+            text: text.to_string(),
             params: log_entry_params(text, compiled_params),
         });
         if entries.len() >= limit {