fix: multi-segment full coverage misclassified as PartialOverlap

[Zious11]

Summary

Found by Copilot during PR #33 review. Pre-existing issue in insert_segment overlap detection.

Problem

The fully_covered check in FlowDirection::insert_segment (src/reassembly/segment.rs:125-127) only tests if a single existing segment covers [new_start, new_end):

let fully_covered = trimmed_ranges
    .iter()
    .any(|&(es, ee)| es <= new_start && ee >= new_end);

When the new segment is fully covered by the union of multiple adjacent existing segments, fully_covered is false. The gap computation correctly produces an empty gaps vec, but the return logic (line 181-186) reports PartialOverlap instead of Duplicate.

Impact

• segments_inserted stat inflated by 1 for each occurrence (caller counts PartialOverlap as inserted)
• No data corruption — no bytes are actually inserted
• Rare in practice — requires a new segment exactly spanning multiple existing segments with no gap

Fix

After the gap computation, check !had_gap && !has_conflict → return Duplicate instead of falling through to PartialOverlap:

return if !had_gap && has_conflict {
    InsertResult::ConflictingOverlap
} else if !had_gap {
    InsertResult::Duplicate  // fully covered by union of segments
} else if truncated {
    InsertResult::Truncated
} else {
    InsertResult::PartialOverlap
};