feat: migrate the proxy HTTP stack to josexy/xhttp and josexy/net

[josexy]

Summary

• migrate all HTTP-facing code and examples from net/http to github.com/josexy/xhttp
• replace direct golang.org/x/net usage with github.com/josexy/net
• update github.com/josexy/websocket to use the same HTTP and networking stack
• preserve request and response header blocks, wire order, and trailers across HTTP/1.1, HTTP/2, and h2c
• automatically capture inbound HTTP/2 connection and request profiles and replay them through the upstream transport, including SETTINGS, WINDOW_UPDATE, priority metadata, and pseudo-header order
• extend the dumper diagnostics and add unit, benchmark, h2c, and TLS HTTP/2 end-to-end coverage

HTTP/2 profile capture and propagation are handled internally and require no additional runtime configuration.

Breaking changes

Public APIs that previously used net/http types now use github.com/josexy/xhttp types.

Consumers must update their HTTP imports, handlers, interceptors, request/response helpers, and server entry points accordingly.

Dependency revisions

• github.com/josexy/xhttp@b930f9a
• github.com/josexy/net@c0d6b15b
• github.com/josexy/websocket@4bb5473

Testing

• go test ./...
• GitHub Actions Test (1.26.x) passes