BlockStore: fix correctness issues in block storage layer (#996)

1. **Assert invariant in `RemoveActiveWriteBlock`** — `erase(std::find(...))` was UB if the invariant ever broke. Now asserts the iterator before erasing.
2. **Single atomic delta in `SetMetaData`** — was `+= new; -= old` as two atomic ops, briefly inflating `TotalSize()` for concurrent readers. Collapsed into one `fetch_add`.
3. **Consistent `IncludeBlocks` / `IncludeBlock`** — `IncludeBlocks` asserted on duplicate keys while `IncludeBlock` silently skipped. Made both tolerant; also made the `reserve` call additive so a second call can't shrink the capacity request.
4. **Replace `operator[]` reads with `find` on `m_ChunkBlocks`** — `tsl::robin_map::operator[]` default-inserts; several read-intent lookups could produce ghost null entries if invariants broke (especially on compaction rollback paths).
5. **Bound `GetChunk` against actual file size** — `m_IoBuffer.GetSize()` is the mapped capacity (block size, e.g. 256 MiB), not written bytes. Requests inside the mapped region but past the real EOF returned views over zero-filled memory. Now bounds against `FileSize()`.

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