Adding Stellar CAIP-2 Namespace 

Reference: https://namespaces.chainagnostic.org/stellar/caip2

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<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Low Risk**
> Low risk: adds a new enum member and changelog entry with no
behavioral logic changes beyond widening accepted known CAIP namespaces
at the type level.
> 
> **Overview**
> Adds `stellar` as a new `KnownCaipNamespace` option in
`src/caip-types.ts`, allowing `KnownCaipNamespacedChainId` to represent
Stellar chains.
> 
> Updates the `CHANGELOG.md` *Unreleased* section to document the new
Stellar CAIP namespace addition.
> 
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This is the release candidate for version 11.11.0.

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