veb: support reusable takeover connections

[guweigang]

Summary

This adds a reusable takeover mode for veb/fasthttp connections.

• replace the boolean takeover state with explicit takeover modes in veb and fasthttp
• add Context.takeover_conn_reusable() for handlers that write one complete response themselves and then return the connection to the server read loop
• teach the fasthttp Linux and BSD backends to keep reusable takeover connections alive, while still honoring shutdown and Connection: close
• add fasthttp and veb tests covering reusable takeover on a persistent TCP connection

Motivation

Context.takeover_conn() is useful for SSE and WebSocket-style flows where the handler fully owns the socket. Some streaming handlers write exactly one complete HTTP response, including a clear body boundary, and then want the server to continue serving later requests on the same connection.

The new reusable takeover mode represents that case directly without forcing the handler to close the connection.

Validation

./v test vlib/fasthttp vlib/veb/tests/persistent_connection_test.v vlib/veb/sse/sse_test.v vlib/veb/tests/veb_test.v
git diff --cached --check