Update Version.swift and CHANGELOG to v1.1

[nficano]

Swift SDK wire version is hardcoded to "1.0":

• Sources/ARCP/Version.swift — change "1.0" → "1.1"
• Tests/ARCPTests/EnvelopeTests.swift — 3 fixtures assert "arcp": "1.0"; spec says envelope arcp field is "1" (major). Either update fixtures to "1" or document as test data
• CHANGELOG.md initial-release entry — reword "v1.0" → "v1.1 (additive over v1.0)"

Verify: grep -rEn 'v1\.0|"1\.0"' Sources/ Tests/ CHANGELOG.md README.md returns only intentional refs.

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)