feat(encryption): implement the FEE wire-format foundation

[BravoNatalie]

Description

Implement the first layer of @filoz/filecoin-encryption-envelope: COSE envelope serialization, validation, and chunk-layout primitives. This work defines how FEE objects are represented and parsed, but does not include AEAD encryption or key wrapping.

Proposed solution

Add strict COSE_Encrypt0 and COSE_Encrypt handling with detached ciphertext, protected headers, recipient validation, authenticated-data construction, chunk arithmetic, and nonce derivation.

Done criteria

• Encode and decode COSE tags 16 and 96
• Preserve protected-header bytes for AAD
• Validate FEE headers, crit, IVs, recipients, and algorithm placement
• Support plaintext_length and chunk-layout calculations
• Derive chunk nonces from the base nonce, chunk index, and final flag
• Reject malformed, ambiguous, or unsupported CBOR
• Enforce envelope, object, chunk-count, chunk-size, and nesting limits
• Add byte-exact fixtures and malformed-input regression tests