feat(e2ee): add end-to-end HTTP payload encryption package

[vitalvas]

Summary

Adds e2ee, a new package implementing end-to-end encryption for HTTP API payloads per draft-vasylenko-e2ee-http. It protects request and response bodies independently of, and in addition to, TLS, so TLS-terminating intermediaries (CDNs, reverse proxies, load balancers) cannot read or tamper with payloads.

What's included

• Crypto (Go standard library only): X25519 key agreement (RFC 7748), HKDF-SHA256 directional request/response keys (RFC 5869), AES-128/192/256-GCM envelope nonce || ciphertext || tag (NIST SP 800-38D).
• E2EE-Session structured field (RFC 9651) with deterministic canonical serialization bound into the AES-GCM AAD, preventing intermediary tampering and response substitution.
• Key discovery: /.well-known/encryption-keys JSON key set (ServerKeySet.Handler, FetchKeySet), validity windows, max_skew, fingerprints, and overlapping-key rotation.
• Client: encrypting http.RoundTripper, fresh ephemeral key per request by default with an optional reusable Session, and response echo validation.
• Server: mux middleware implementing the draft's full validation order; pluggable ReplayCache interface with an in-memory default.
• Errors: RFC 9457 Problem Details with urn:ietf:params:e2ee:error:<code> types; internal errors never leaked.

Reuse

Reuses sibling kasper packages instead of reimplementing them: muxhandlers.ProblemDetails / WriteProblemDetails for RFC 9457, and mux.ResponseJSON + mux.ContentType* constants for JSON responses and media types. No new external dependencies (only the already-present github.com/google/uuid).

Testing

• 100% statement coverage, race-clean.
• yake run passes: golangci-lint 0 issues and all Go policy checks (string-concat, stdlib-wrapper, signature complexity, composite-literal, test-file naming, coverage).

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