docs(readme): bust camo cache via path change on downloads badge

Adds .svg suffix to the downloads-badge URL path. Two effects:
1. The full URL hash (which camo keys on) changes, so GitHub's image proxy treats this as a brand-new URL and fetches fresh from shields.io instead of serving the previously-cached "invalid" SVG (camo's cache TTL was 30 min, locked in from the original transient shields.io flake earlier today).
2. .svg is a documented shields.io path suffix — verified via curl that `/total.svg?label=...` returns the same SVG content as `/total?label=...`.

The previous cacheSeconds=60 → cacheSeconds=300 dance also produces a new URL hash, but camo had separately cached the cacheSeconds=60 variant during a shields.io flake, so the user kept seeing invalid. New path = guaranteed fresh fetch.

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&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=60)](https://github.com/therealaleph/MasterHttpRelayVPN-RUST/releases)
+[![Downloads](https://img.shields.io/github/downloads/therealaleph/MasterHttpRelayVPN-RUST/total.svg?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)