feat: harden TLS 1.3 client boundaries and certificate parsing

[takeokunn]

Summary

• Harden the QUIC TLS boundary to the contract levels :initial, :handshake, and :application; accept legacy :1-rtt as an alias.
• Use crypto-kit CSPRNG defaults for ClientHello random and session ID, and reject invalid ServerHello choices and downgrade sentinels.
• Harden X.509 parsing and verification with strict DER/PEM validation, bounded certificate input/depth/chain size, critical Name Constraints rejection for unsupported GeneralName types, and matching TBSCertificate/outer signature algorithms.
• Run native Nix checks and evaluate all flake systems with nix flake check --all-systems --no-build in CI.

Verification evidence

• Local nix-check.sh -> nix flake check --all-systems: exit 0.
• Local targeted X.509/security tests: exit 0 (basic-pass), including strict DER length/depth/high-tag, invalid PEM, chain limits, critical Name Constraints, and signature algorithm mismatch rejection.
• New head: 6fcf680, signed; git diff --check: exit 0.
• Ubuntu 24.04 CI pass: run 37094402348, job 111121252665; run 37094400477, job 111121246585.
• Lisp CI pass: run 37094402160, job 111121251526; run 37094400474, job 111121246393.

The macOS jobs remain queued on GitHub; the workflow macOS matrix and QUIC public API were not changed.

[takeokunn]

追加更新（commit c66ae44）: tls-client-read が受信 fatal alert を tls-client-alert-error として通知し、close_notify で closed 遷移、受信 KeyUpdate を provider callback にルーティングするようになりました。nix-check.sh は exit 0、Nix log は 8 groups passed; RFC 8448 Simple 1-RTT: PASS です。残課題は provider-independent full handshake driver と OpenSSL 3 suites x 2 groups の E2E matrix です。

[takeokunn]

追加更新（commit 35fd982）: src/client-driver.lisp に provider の handshake callback に依存しない進行ドライバを追加しました。ClientHello/key_share、HRR、ServerHello、handshake secret、server Finished 検証、client Finished 送出を実装し、driver startup テストを追加。nix-check.sh は exit 0、Nix log は 9 groups passed; RFC 8448 Simple 1-RTT: PASS。残りは record-layer transport 統合、証明書/CertificateVerify の実 handshake 接続、OpenSSL E2E matrix です。