shadow: drop obsolete quinn-udp patch (fixes auto-rebase Cargo.lock conflict)

[ch4r10t33r]

What & why

The Shadow Branch Auto-Rebase workflow has been failing on every push to main (example run) with a rust/Cargo.lock conflict while replaying the shadow patch commit onto main.

Root cause: the shadow patch carried a vendored quinn-udp fallback (rust/patch/quinn-udp + [patch.crates-io] + hand-edited rust/Cargo.lock/Cargo.toml). That patch is now obsolete — zeam's QUIC networking is pure zig-libp2p (zquic), not rust/quinn, and the Shadow recvmmsg → recvfrom fallback lives in zig-libp2p as of v0.2.65 (main already pins v0.2.67) and engages automatically on ENOSYS. The hand-edited rust/Cargo.lock was the recurring conflict, and it bought nothing.

Change

Rewrites the shadow patch commit to drop all rust/quinn-udp content, keeping only the still-relevant Shadow-only bits:

• ❌ removed: rust/patch/quinn-udp/*, [patch.crates-io] quinn-udp in rust/Cargo.toml, the rust/Cargo.lock edit
• ✅ kept: docs/shadow-simulation.md (updated to describe the zig-libp2p Shadow path instead of quinn-udp) and the lean-quickstart shadow-automation tip

With the rust patch gone, the shadow branch no longer touches rust/Cargo.lock, so the auto-rebase applies cleanly going forward.

⚠️ How to merge

shadow is a single-patch, force-pushed branch (the auto-rebase rewrites it), so this can't land as a normal merge commit — that would keep the old conflicting patch in history. Force-update shadow to this branch's tip instead:

git fetch origin
git push --force-with-lease origin origin/fix/shadow-strip-quinn-udp:shadow

(or I can do it once you've reviewed). This PR is the review artifact for that replacement; the noisy file diff vs shadow is just main catching up — the only substantive change is dropping the rust/quinn-udp patch.

[GrapeBaBa]

@ch4r10t33r No need it right now, shadow branch already catchup latest main