Projecting: handle multiple prooph's streams

[jlabedo]

Why is this change proposed?

Handle projecting from multiple prooph's stream

Description of Changes

• Implemented EventStoreMultiStreamSource as a composition over EventStoreGlobalStreamSource instances.
• Per stream, a small page is loaded and events are interleaved globally by:
  • created_at column
  • configured stream order (tie‑breaker)
  • no ascending (final tie‑breaker)
• Combined projection position is encoded as stream=position:g1,g2;....

The requested batch size is handled on a best-effort basis: it may be smaller or larger depending on how events are distributed across streams.

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