Implement iOS HMAC actuals (CommonCrypto) — iOS Phase 2

[ahmetabdullahgultekin]

Why: The KMP TOTP engine's iOS HMAC actuals are TODO('iOS HMAC via CommonCrypto') stubs in HmacPlatform.ios.kt; iOS is Phase 2 and blocked on Apple Developer enrollment (no iosApp/ module exists yet). Tracked in docs/plans/CLIENT_APPS_PARITY.md (parent repo).

Done when: the three iOS HMAC actuals (SHA1/256/512) are implemented via CommonCrypto and the shared TOTP engine builds/passes its RFC 6238 vectors on iOS.

source: shared/src/iosMain/kotlin/com/fivucsas/authenticator/totp/HmacPlatform.ios.kt:4

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Migrated from shared/src/iosMain/kotlin/com/fivucsas/authenticator/totp/HmacPlatform.ios.kt:4.