add realip, requestlog, recoverer middlewares

Initial implementation of lightmux-contrib, a sibling module to
lightmux that hosts opinionated middlewares with one sub-package per
middleware:

- realip: resolves the originating client IP from CF-Connecting-IP,
  True-Client-IP, X-Real-IP, or X-Forwarded-For. Optional peer-CIDR
  allowlist via netip.Prefix.
- requestlog: emits a structured http.request record (method, path,
  status, duration, client) per request via splinter.
- recoverer: catches panics, wraps with errx under op "recoverer",
  logs with stack, and writes a 500 response.

Each package exposes a single New(...) constructor returning
func(http.Handler) http.Handler. The contrib module intentionally
does not import lightmux — middlewares interoperate via the standard
stdlib middleware shape.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.mod

@@ -0,0 +1,8 @@
+module git.juancwu.dev/juancwu/lightmux-contrib
+
+go 1.26.2
+
+require (
+	git.juancwu.dev/juancwu/errx v0.1.0
+	git.juancwu.dev/juancwu/splinter v0.1.0
+)