refactor(streaming): P2/E1 — delete legacy proxy path (ProxyCommand)

Phase E (ADR 0003), step 1: proxy live streams are now daemon-only. The
daemon owns the proxy puller (registered + off-air-probed in live.php), so
the legacy producer + relay are dead code and are removed:

- delete Cli/Commands/ProxyCommand.php (the XC_VMProxy producer).
- remove StreamProcess::startProxy and ProcessManager::startProxy (only
  caller was live.php).
- live.php: drop the startProxy branch in the process block and the
  AF_UNIX socket relay in the proxy delivery arm. A proxy stream whose
  daemon is unreachable ($rFanout false) now shows not-on-air — the
  keepalive restarts the daemon in ~2s — instead of falling back to the
  legacy producer.

This removes the proxy fallback: the daemon is required for proxy delivery
(its own crash resilience is the keepalive). Non-proxy legacy stays for now.
PHPStan clean, 443 tests.

Refs ADR 0003 (Phase E).

src/Core/Process/ProcessManager.php

@@ -507,18 +507,6 @@ class ProcessManager {
         return true;
     }
 
-    /**
-     * Start a proxy process in background.
-     *
-     * Extracted from ProcessManager::startProxy().
-     *
-     * @param int $streamID
-     * @return bool
-     */
-    public static function startProxy($streamID) {
-        shell_exec(PHP_BIN . ' ' . MAIN_HOME . 'console.php proxy ' . intval($streamID) . ' >/dev/null 2>/dev/null &');
-        return true;
-    }
 
     // ───────────────────────────────────────────────────────────
     //  Utility