Production-ready HTTP server implementing app.Component with automatic
/-/liveness and /-/readiness endpoints. Readiness checks are chainable
and run concurrently. Includes request logging and tracing middleware.

Replace gorilla/mux with stdlib net/http as the default router. Expose
a Router interface so the underlying implementation can be swapped
without breaking consumers. No framework types in the public API.

Changes:
- Define Router interface with Handle, HandleFunc, Use, ServeHTTP
- Default implementation uses Go 1.22+ net/http.ServeMux with method
  pattern matching (GET /-/liveness, etc.)
- Add Config.Router for custom Router injection
- Middleware signatures use func(http.Handler) http.Handler (stdlib)
- Health check endpoints still registered automatically
- Tests exercise the Router interface, not a concrete framework
- New tests for Handle, Use middleware, and custom Router injection

Execute readiness checks using goroutines so a slow check does not block
others, matching the documented concurrent behavior. Also log JSON
encoding failures in writeJSON for debugging.

Use sync.Once for thread-safe lazy initialization of the handler chain
in defaultRouter.ServeHTTP, preventing concurrent goroutines from
building the chain simultaneously. Also use the server's configured
logger in writeJSON instead of slog.Default() for consistency.

Address AGENTS.md review findings:
- Add Name field to Config with default "httpserver" for component
  identification in logs and errors
- Use net.ListenConfig.Listen(ctx) in Start to honour context
  cancellation during bind
- Wrap Shutdown errors with the component name
- Fix doc.go claim about logging middleware recording duration
  (it only logs method, path, and status)

Replace the stdlib net/http.ServeMux default with go-chi/chi, exposing
Chi's key features through the Router interface while keeping Chi types
out of the public API (design requirement from team-tasks#4283).

Router interface changes:
- Add method-specific registration: Get, Post, Put, Delete, Patch
- Add route grouping: Group (scoped middleware), Route (path prefix)
- Add sub-router mounting: Mount
- Add per-route middleware: With
- Add error handler customization: NotFound, MethodNotAllowed
- Add URLParam helper for path parameter extraction

Architecture changes:
- Default Router backed by chi.Mux (wrapped in unexported chiRouter)
- Built-in middleware (tracing, logging) registered via Use before user
  routes, running inside chi's routing context for access to route
  patterns
- Health endpoints (/-/liveness, /-/readiness) handled by Server.ServeHTTP
  directly, outside the user's route tree
- Tracing middleware uses chi.RouteContext for low-cardinality span names
  (e.g. "GET /users/{id}" instead of "GET /users/42")
- Compile-time app.Component assertion added

Removed:
- routerbench standalone benchmark module (Chi/Gorilla/stdlib comparison
  served its purpose during the investigation phase)

feat(v2/httpserver): Add HTTP server with built-in health probes

See merge request !339



Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: dbarrett <dbarrett@gitlab.com>