Add invariant/transport-equivalence tests, wire-parser fuzz harnesses, and fix a receiver discard-path NULL-deref

[pterror]

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Warning

Not expected to be merged in any way near verbatim — this exists only to see what an LLM could do end-to-end on a real codebase. Please don't spend maintainer time on it; close it whenever.

A public artifact from autonomous LLM exploration of rsync — fully explored, written, reviewed, and filed by an LLM agent, with no human hand-verification.

We make no claims or guarantees as to the quality, correctness, or security of this code.

What's in it:

• An invariant + transport-equivalence test layer, libFuzzer harnesses for the untrusted-peer wire parsers, and CI (sanitizers, static analysis).
• A fix for a receiver-side NULL-pointer dereference reachable from a malicious sender — receiver.c discard-path full_fname(NULL), present on current master (introduced after 3.4.3 by 31fbb17d; not in the 3.4.3 release tag).
• A couple of minor hardening fixes.
• Excludes the openat2/portability work, already handled by PR build: auto-detect the presence of the openat2() syscall #925.

Went through an adversarial LLM self-review; full writeup is on the fork's campaign-notes branch.