docs(readme): cache-bust camo on the release+downloads badges

Both badges resolved correctly when fetched directly from shields.io but GitHub's image proxy (camo.githubusercontent.com) was still serving the previously-invalid cached SVG to README viewers. Adding `cacheSeconds=300` changes the URL hash camo keys on, forcing a fresh fetch, AND tells shields.io to set a 5-minute Cache-Control on the SVG so future invalidations propagate quickly.

Caught a separate shields.io quirk in passing: parameter ordering on `/github/downloads/...` is significant — `?label=X&logo=Y` resolves, `?logo=Y&label=X` returns invalid (reproduced via curl). Order in README is the working one. The `?sort=semver` parameter remains the original release-badge breaker (#previous fix).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: therealaleph

README.md

@@ -1,7 +1,7 @@
 # mhrv-rs — bypass censorship for free, with your own Google account
 
-[![Latest release](https://img.shields.io/github/v/release/therealaleph/MasterHttpRelayVPN-RUST?display_name=tag&logo=github&label=release&color=blue)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/releases/latest)
-[![Downloads](https://img.shields.io/github/downloads/therealaleph/MasterHttpRelayVPN-RUST/total?label=downloads&logo=github)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/releases)
+[![Latest release](https://img.shields.io/github/v/release/therealaleph/MasterHttpRelayVPN-RUST?display_name=tag&logo=github&label=release&color=blue&cacheSeconds=300)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/releases/latest)
+[![Downloads](https://img.shields.io/github/downloads/therealaleph/MasterHttpRelayVPN-RUST/total?label=downloads&logo=github&cacheSeconds=300)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/releases)
 [![CI](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/actions/workflows/release.yml/badge.svg)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/actions/workflows/release.yml)
 [![License: MIT](https://img.shields.io/github/license/therealaleph/MasterHttpRelayVPN-RUST?color=blue)](LICENSE)
 [![Stars](https://img.shields.io/github/stars/therealaleph/MasterHttpRelayVPN-RUST?style=flat&logo=github)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/stargazers)