You are working on `git.codelab.vc/pkg/httpx`, a Go 1.24 HTTP client/server library with zero external dependencies.

## Architecture

- Client middleware: `func(http.RoundTripper) http.RoundTripper` — compose with `middleware.Chain`
- Server middleware: `func(http.Handler) http.Handler` — compose with `server.Chain`
- All configuration uses functional options pattern (`WithXxx` functions)
- Chain order for client: Logging → User MW → Retry → Circuit Breaker → Balancer → Transport

## Package structure

- `httpx` (root) — Client, request builders (NewJSONRequest, NewFormRequest), error types
- `middleware/` — client-side middleware (Logging, Recovery, Auth, Headers, RequestID)
- `retry/` — retry middleware with exponential backoff and Retry-After support
- `circuitbreaker/` — per-host circuit breaker (sync.Map of host → Breaker)
- `balancer/` — load balancing with health checking (RoundRobin, Weighted, Failover)
- `server/` — Server, Router, server middleware (RequestID, Recovery, Logging, CORS, RateLimit, MaxBodySize, Timeout), response helpers (WriteJSON, WriteError)
- `internal/requestid/` — shared context key (avoids circular import between server and middleware)
- `internal/clock/` — deterministic time for tests

## Code conventions

- Zero external dependencies — stdlib only, do not add imports outside the module
- Functional options: `type Option func(*options)` with `With<Name>` constructors
- Test with stdlib only: `testing`, `httptest`, `net/http`. No testify/gomock
- Client test helper: `mockTransport(fn)` wrapping `middleware.RoundTripperFunc`
- Server test helper: `httptest.NewRecorder`, `httptest.NewRequest`, `waitForAddr(t, srv)`
- Thread safety with `sync.Mutex`, `sync.Map`, or `atomic`
- Use `internal/clock` for time-dependent tests, not `time.Now()` directly
- Sentinel errors in sub-packages, re-exported as aliases in root package

## When writing new code

- Client middleware → file in `middleware/`, return `middleware.Middleware`
- Server middleware → file in `server/middleware_<name>.go`, return `server.Middleware`
- New option → add field to options struct, create `With<Name>` func, apply in constructor
- Do NOT import `server` from `middleware` or vice versa (use `internal/requestid` for shared context)
- Client.Close() must be called when using WithEndpoints() (stops health checker goroutine)
- Request bodies must have GetBody set for retry — use NewJSONRequest/NewFormRequest

## Commands

```bash
go build ./...           # compile
go test ./...            # test
go test -race ./...      # test with race detector
go vet ./...             # static analysis
```