fix: sandbox network responsiveness under port-binding apps

Running port-binding applications (Jupyter, http.server, NextJS) inside
sandboxes caused severe PTY sluggishness and proxy navigation errors.

Root cause: the CP sandbox proxy and Connect RPC pool shared a single
HTTP transport. Heavy proxy traffic (Jupyter WebSocket, REST polling)
interfered with PTY RPC streams via HTTP/2 flow control contention.

Transport isolation (main fix):
- Add dedicated proxy transport on CP (NewProxyTransport) with HTTP/2
  disabled, separate from the RPC pool transport
- Add dedicated proxy transport on host agent, replacing
  http.DefaultTransport
- Add dedicated envdclient transport with tuned connection pooling
- Replace http.DefaultClient in file streaming RPCs with per-sandbox
  envd client

Proxy path rewriting (navigation fix):
- Add ModifyResponse to rewrite Location headers with /proxy/{id}/{port}
  prefix, handling both root-relative and absolute-URL redirects
- Strip prefix back out in CP subdomain proxy for correct browser
  behavior
- Replace path.Join with string concat in CP Director to preserve
  trailing slashes (prevents redirect loops on directory listings)

Proxy resilience:
- Add dial retry with linear backoff (3 attempts) to handle socat
  startup delay when ports are first detected
- Cache ReverseProxy instances per sandbox+port+host in sync.Map
- Add EvictProxy callback wired into sandbox Manager.Destroy

Buffer and server hardening:
- Increase PTY and exec stream channel buffers from 16 to 256
- Add ReadHeaderTimeout (10s) and IdleTimeout (620s) to host agent
  HTTP server

Network tuning:
- Set TAP device TxQueueLen to 5000 (up from default 1000)
- Add Firecracker tx_rate_limiter (200 MB/s sustained, 100 MB burst)
  to prevent guest traffic from saturating the TAP

internal/sandbox/manager.go

@@ -53,6 +53,15 @@ type Manager struct {
 
 	autoPausedMu  sync.Mutex
 	autoPausedIDs []string
+
+	// onDestroy is called with the sandbox ID after cleanup completes.
+	// Used by ProxyHandler to evict cached reverse proxies.
+	onDestroy func(sandboxID string)
+}
+
+// SetOnDestroy registers a callback invoked after each sandbox is cleaned up.
+func (m *Manager) SetOnDestroy(fn func(sandboxID string)) {
+	m.onDestroy = fn
 }
 
 // sandboxState holds the runtime state for a single sandbox.
@@ -314,6 +323,10 @@ func (m *Manager) Destroy(ctx context.Context, sandboxID string) error {
 		slog.Warn("snapshot cleanup error", "id", sandboxID, "error", err)
 	}
 
+	if m.onDestroy != nil {
+		m.onDestroy(sandboxID)
+	}
+
 	slog.Info("sandbox destroyed", "id", sandboxID)
 	return nil
 }