Fix connection lifecycle, SSH trust and configuration performance

[andre487]

Fix seven issues found in the repository review: stopped HTTPS dialers now close active HTTP/1.1 and direct sockets, cancelled CONNECT requests and saturated admission waits finish promptly, and silent SSH keepalives invalidate only their own session after ten seconds. Cached proxy addresses are refreshed on refused/unreachable upstream connections without displaying a blocking warning; a newer cached address and stale-session callbacks are preserved.

SSH key approval now checks the captured hostname, port and previous pin inside the atomic write, including notification restoration. SOCKS5 UDP reads reuse an association buffer. Single-profile lookup decrypts only that profile, cold screen reads run on the existing configuration dispatcher, and editable controls appear after loading. Update downloads retain the current APK plus one predecessor and clean abandoned partial files. Updated the README and both update guides.

Validation:

• Full native suite with race detector: 208 tests passed, including CONNECT cancellation, established tunnel lifetime, connection admission, silent SSH keepalives and a UDP allocation budget.
• Regression coverage for changed SSH endpoints/pins, selective decryption, bounded DNS refresh and stale callbacks, slow UI reads, and update cache retention.
• Android checks: 275 JVM tests passed; lint, debug build and unsigned release build passed.
• Local real GOST/OpenSSH integration could not build its fixture: Docker Hub's Alpine authentication endpoint returned a malformed HTTP response before any proxy scenario started. Required CI also exercises these containers and API 26/API 35 emulator scenarios.

APK artifacts

• Download debug APK — signed with the disposable Android debug key
• Download unsigned release APK — verified unsigned

Built from commit ca819ceb by CI run #295.
Artifacts expire after 14 days. Neither APK uses the MegaProxy release key.