fix(deps): bump quinn-proto past RUSTSEC-2026-0185 (lockfile-only) (#295)

## Summary
- Bumps `quinn-proto` 0.11.14 -> 0.11.15 via `cargo update -p
quinn-proto` to clear
[RUSTSEC-2026-0185](https://rustsec.org/advisories/RUSTSEC-2026-0185)
(high, 7.5), which is failing the Rust CI audit job and the standalone
Cargo Audit workflow.
- Lockfile-only change: `Cargo.lock` is the only file touched (verified
with `git status`).

## Verification
- `grep -A2 'name = "quinn-proto"' Cargo.lock` -> `version = "0.11.15"`
- Local `cargo audit` (advisory DB 1146 entries) exits 0:
RUSTSEC-2026-0185 cleared. The remaining RUSTSEC-2026-0190 (`anyhow`,
unsound) is an allowed WARNING, not a failure, and is intentionally left
alone.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

Author: hyperpolymath