perf: Use HashMap for OffsetSnapshot lookups

[tomverelst]

Problem

OffsetSnapshot::get_opt() in bridge-core/src/snapshot/mod.rs does linear search through the vector. For large snapshots with many consumer groups/topics/partitions, this is O(n) per lookup.

Suggestion

Use a HashMap<(ConsumerGroup, Topic, Partition), Offset> for O(1) lookups instead of iterating through the vector.

// Current: O(n) linear search
pub fn get_opt(&self, group: &str, topic: &str, partition: i32) -> Option<&OffsetRecord> {
    self.0.iter().find(|r| r.consumer_group == group && r.topic == topic && r.partition == partition)
}

// Suggested: O(1) HashMap lookup

Impact

Improves performance for migrations involving many consumer groups or topics.