Align NIP-46 validators with canonical names

[AustinKelsay]

Part of #154
Blocked by: #156

Goal

Give NIP-46 boolean validators NIP-specific names to avoid colliding with security-validator throwing validators, keep old names as deprecated Compatibility Aliases, and route private-key boolean checks through canonical key-validation semantics.

Acceptance

• New names exist (e.g. isValidNip46EventContent, isValidNip46PrivateKey, …) for the previously colliding helpers.
• Old exported names remain and are @deprecated aliases.
• isValidNip46PrivateKey / deprecated validatePrivateKey use isValidPrivateKey from key-validation (secp256k1), not hex-length-only.
• Public NIP-46 validator suites stay green; update call sites inside src/nip46 to prefer new names.

Notes

AFK. Behavior change for invalid-hex-length-but-invalid-curve keys is intentional hardening and covered by tests.

[AustinKelsay]

Implemented on feature/cleanup-tier-ab-api-surface and included in PR #163 targeting staging. Spec #154.