chore: caip type for stellar

[khanti42]

Adding Stellar CAIP-2 Namespace

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

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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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