Replace HashSet<ulong> _deleted in MsgBlock with SequenceSet — a sorted-range
list that compresses contiguous deletions into (Start, End) intervals. Adds
O(log n) Contains/Add via binary search on range count, matching Go's avl.SequenceSet
semantics with a simpler implementation.
- Add SequenceSet.cs: sorted-range compressed set with Add/Remove/Contains/Count/Clear
and IEnumerable<ulong> in ascending order. Binary search for all O(log n) ops.
- Replace HashSet<ulong> _deleted and _skipSequences in MsgBlock with SequenceSet.
- Add secureErase parameter (default false) to MsgBlock.Delete(): when true, payload
bytes are overwritten with RandomNumberGenerator.Fill() before the delete record is
written, making original content unrecoverable on disk.
- Update FileStore.DeleteInBlock() to propagate secureErase flag.
- Update FileStore.EraseMsg() to use secureErase: true via block layer instead of
delegating to RemoveMsg().
- Add SequenceSetTests.cs: 25 tests covering Add, Remove, Contains, Count, range
compression, gap filling, bridge merges, enumeration, boundary values, round-trip.
- Add FileStoreTombstoneTrackingTests.cs: 12 tests covering SequenceSet tracking in
MsgBlock, tombstone persistence through RebuildIndex recovery, secure erase
payload overwrite verification, and FileStore.EraseMsg integration.
Go reference: filestore.go:5267 (removeMsg), filestore.go:5890 (eraseMsg),
avl/seqset.go (SequenceSet).
