fix: add SimpleKeyringV2 missing exports

[mathieuartu]

Examples

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Note

Low Risk
Low risk: this only updates public exports (and the changelog) without changing keyring behavior, but it can affect consumers’ build/type resolution if they relied on prior export paths.

Overview
Fixes the package entrypoint to export SimpleKeyringV2 and its SimpleKeyringV2Options type from src/index.ts, making the V2 keyring adapter available to consumers via the main module import.

Updates the CHANGELOG.md to document the missing-export fix under Unreleased.

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