chore(deps): bump usrsctp to upstream master@c625e21 (2026-10-07)

[gg582]

What

Bumps the vendored lib/usrsctp submodule:

• Old: fd070e05a7474f38c7fecdf4d4b6005d2547ee00 (Fix build with MinGW, #730)
• New: c625e21950600460cb5c6075616eb1622df2b06a — current HEAD of upstream master (https://github.com/sctplab/usrsctp)

Upstream changes since the old pin (2 commits, both from 2026-10-07):

• c625e21 Removed unused variable "cnt" from SCTP_RESET_STREAMS case of sctp_setopt() (#753)
• bfb2eee Fix cmake_minimum_required warning (#751)

No API changes; no CWIST compat changes needed.

Verification (local)

In a fresh worktree based on origin/dev (9b058b62), with lib/usrsctp checked out at the new upstream HEAD and all other submodules symlinked to the prebuilt copies in the main checkout:

make test_webrtc CWIST_WEBTRANSPORT=server -j$(nproc)

This rebuilt libusrsctp.a from the new sources via CMake, rebuilt libcwist.a (including src/net/webrtc/sctp.o against the new headers), linked test_webrtc, and ran it:

./test_webrtc
test_webrtc: all assertions passed

Exit code 0. (CWIST_WEBTRANSPORT=server matches the flags the prebuilt vendored libs were compiled with; the prebuilt lsquic lacks lsquic_wt.h, so the dev-default client,server role set cannot be used in this local setup.)

Full CI runs on this PR.