[hadashiA]

What

• DefaultStorageFactory now uses MemoryStream.TryGetBuffer to hand the stream's own buffer to InMemoryPageLoader, falling back to ToArray() only when the buffer is not exposable (e.g. new MemoryStream(bytes)).
• New overload ReadOnlyDatabase.OpenAsync(ReadOnlyMemory<byte> data) opens a database directly from a caller-owned buffer with no Stream and no copy. The catalog parser is fed through a new internal seekable ReadOnlyMemoryStream.
• InMemoryPageLoader now takes ReadOnlyMemory<byte> instead of Memory<byte> — it never writes to the buffer. Source-compatible for existing callers (byte[] / Memory<byte> convert implicitly).

Why

Opening a fully in-memory database previously always paid a second full-size allocation (ms.ToArray()), doubling peak memory — noticeable for the embedded/game use cases where the whole file is already in memory.

Notes for review

• Zero-copy paths alias the live buffer, so the stream/memory must not be mutated while the database is open. This is noted on the overload's XML doc and at the TryGetBuffer site.
• ReadOnlyMemoryStream implements only what the catalog parser needs (Read/ReadAsync/ReadByte/Seek); writes throw NotSupportedException.
• Tests cover all three paths: the memory overload, the exposable-buffer stream path, and the ToArray() fallback. 98/98 tests pass (net10.0).

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