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>

Taskfile.yml

@@ -0,0 +1,43 @@
+version: '3'
+
+tasks:
+  default:
+    desc: Run vet and tests
+    cmds:
+      - task: check
+
+  install:tools:
+    desc: Install development tools (tparse for prettier test output)
+    cmds:
+      - go install github.com/mfridman/tparse@latest
+
+  test:
+    desc: Run tests with prettier output via tparse
+    cmds:
+      - set -o pipefail && go test ./... -json -cover | tparse -all
+
+  test:race:
+    desc: Run tests under the race detector
+    cmds:
+      - set -o pipefail && go test ./... -race -json | tparse -all
+
+  vet:
+    desc: Run go vet
+    cmds:
+      - go vet ./...
+
+  fmt:
+    desc: Format Go source files
+    cmds:
+      - gofmt -w .
+
+  tidy:
+    desc: Tidy go.mod
+    cmds:
+      - go mod tidy
+
+  check:
+    desc: Run vet and tests
+    cmds:
+      - task: vet
+      - task: test