[emmanuelgautier]

Closes #10.

Adds smuggling.ambiguous_framing, an aggressive-suite check (runs only with --aggressive) that tests the precondition for every classic request-smuggling attack: whether the proxy rejects RFC 9112-violating ambiguous message framing. Introduces a raw sender (smugprobe) since net/http sanitizes malformed framing before it reaches the wire.

It sends four framings and flags any the proxy accepts (2xx/3xx) instead of rejecting (4xx/5xx or dropped connection):

• conflicting duplicate Content-Length (CL.CL)
• Content-Length + Transfer-Encoding: chunked (CL.TE / TE.CL)
• duplicate Transfer-Encoding with a non-chunked final encoding (TE.TE)
• whitespace-obfuscated header name (Transfer-Encoding : chunked)

Non-destructive: only benign, bounded, self-contained bodies, each on its own Connection: close socket. It never attempts the actual desync (which could poison another client's connection) — it reports purely on accept-vs-reject.

Checklist coverage

• CL.TE, TE.CL, TE.TE classic variants (framing acceptance tested)
• Duplicate/conflicting Content-Length or Transfer-Encoding rejection behavior
• H2.CL / H2.TE on HTTP/2→1.1 downgrade paths — deferred; ties to the HTTP/2 protocol work (HTTP/2 & HTTP/3 Protocol-Specific #13)
• Raw H2C smuggling — deferred (needs HTTP/2 framing control)
• Connection-reuse boundary isolation — deferred

Deferred items are documented in docs/checks/smuggling.mdx. Includes registration and a category test (permissive vs. strict raw-server stand-ins).