Document why MapChunks doesn't allocate a trailing zero-length sentinel

Upstream Java's MMapDirectory.map allocates nrBuffers = floor + 1 and
tolerates a final 0-byte ByteBuffer at `buffers[N]`. That sentinel
exists because ByteBufferIndexInput's read loop unconditionally
advances its buffer cursor past the end of each buffer, so indexing
`buffers[N]` must remain valid after reading the last byte of a file
whose length is a whole multiple of chunkSize.

Our ReadInternal bounds-checks `pos + len` before indexing and only
advances `chunkIdx` while `len > 0`, so no sentinel is needed. Also,
MemoryMappedFile.CreateViewAccessor rejects a zero-length view, which
would force a special-case code path anyway.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: paulirwin

src/Lucene.Net/Store/MMapDirectory.cs

@@ -808,6 +808,18 @@ private static Chunk[] MapChunks(MemoryMappedFile? mmf, long offset, long length
                 }
 
                 long chunkSize = 1L << chunkSizePower;
+                // LUCENENET specific: ceiling-divide, so nChunks covers exactly
+                // the requested range with no trailing empty slot. Upstream
+                // Java (MMapDirectory.map) instead allocates nrBuffers = floor + 1
+                // and tolerates a final 0-byte ByteBuffer — a sentinel required
+                // because ByteBufferIndexInput's read loop unconditionally
+                // advances the buffer cursor past the end of each buffer, so
+                // indexing `buffers[N]` must be valid after the last byte of a
+                // file whose length is a whole multiple of chunkSize. We don't
+                // need that here: ReadInternal bounds-checks `pos + len` before
+                // indexing and only advances `chunkIdx` while `len > 0`. Also,
+                // MemoryMappedFile.CreateViewAccessor rejects a zero-length view,
+                // so a sentinel would require a special-case code path anyway.
                 int nChunks = (int)((length + chunkSize - 1) >> chunkSizePower);
                 var result = new Chunk[nChunks];