Add basic HTTP/2 frame metadata

[keilogic]

Summary

• add basic cleartext HTTP/2 metadata detection when HTTP/1 parsing fails
• recognize the HTTP/2 client connection preface and complete frame headers
• report frame type, flags, stream id, length, and frame count while keeping the existing HTTP/1 behavior intact
• add focused tests for client preface + SETTINGS, server SETTINGS on an HTTP port, and non-HTTP/2 payloads

Fixes #13.

Scope note: this PR does not decrypt HTTPS/TLS traffic. It covers cleartext HTTP/2 (h2c) or already-decrypted HTTP/2 payloads in the existing flow metadata path. TLS keylog decryption is a separate larger feature tracked by #11.

Verification

• py -m py_compile chains\links\http_meta.py chains\links\http_meta_http2_test.py
• py -m pytest chains\links\http_meta_http2_test.py -q
• py -m flake8 chains\links\http_meta.py chains\links\http_meta_http2_test.py
• git diff --check