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/envdclient/client.go

@@ -48,6 +48,13 @@ func (c *Client) BaseURL() string {
 	return c.base
 }
 
+// HTTPClient returns the underlying http.Client used for envd requests.
+// Use this instead of http.DefaultClient when making direct HTTP calls to envd
+// (e.g. file streaming) to avoid sharing the global transport with proxy traffic.
+func (c *Client) HTTPClient() *http.Client {
+	return c.httpClient
+}
+
 // ExecResult holds the output of a command execution.
 type ExecResult struct {
 	Stdout   []byte
@@ -142,7 +149,7 @@ func (c *Client) ExecStream(ctx context.Context, cmd string, args ...string) (<-
 		return nil, fmt.Errorf("start process: %w", err)
 	}
 
-	ch := make(chan ExecStreamEvent, 16)
+	ch := make(chan ExecStreamEvent, 256)
 	go func() {
 		defer close(ch)
 		defer stream.Close()