feat(kernel): admit TCP protocol 6

[kernalix7]

Summary

• harden TCP header parsing for data offsets, options, reserved bits, and ECE/CWR flags
• add allocation-free IPv4 TCP pseudo-header checksum verification
• admit protocol 6 through a stateless fragment/framing/checksum gate with saturating counters and no replies

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace -- -D warnings
• cargo build --workspace
• host unit tests: 4,794 passed
• focused TCP tests: 90 passed
• focused Miri tests: 59 passed
• aarch64 and riscv64 kernel cross-builds
• x86_64, aarch64, and riscv64 QEMU boot integration tests
• parser, checksum, and dispatch mutation proofs
• five-lane goal, QA, quality, security, and context review