SEP-57: Add muxed address event

[ozgunozerk]

Adding Muxed Address Event support to SEP-57

[Copilot]

Pull request overview

This PR updates SEP-57 (T-REX) to describe how transfer events should represent muxed destinations, aligning the document with SEP-41’s muxed-address event semantics.

Changes:

• Bumps SEP-57 metadata (Updated date and Version) and appends a changelog entry for v0.4.0.
• Adds a “MuxedTransfer” transfer-event layout intended to carry a destination muxed ID when applicable.

Suppressed comments (1)

ecosystem/sep-0057.md:518

• The MuxedTransfer example claims to follow SEP-41’s muxed transfer layout, but the field order here is to_muxed_id then amount. SEP-41 specifies the data shape as { amount, to_muxed_id }; keeping the same ordering makes it easier for downstream event decoders to support both the plain and muxed layouts consistently.

    /// The muxed ID of the destination
    pub to_muxed_id: Option<u64>,
    pub amount: i128,
}

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[leighmcculloch]

Heads up that with soroban-sdk v28+ if to_muxed_id: None then the result of the MuxedTransfer event will be identical to the Transfer event and we can start describing events with a single shape.