Add browser extension to ruby_event_store-process_manager

[pjurewicz]

First consumer of the browser extension API from #1951: the process manager gem now ships a browser extension inspecting stateful processes.

Process manager streams follow the ClassName$id convention, so the browser can recognize them and offer a dedicated view:

• a Process state link appears on process manager stream pages
• /process_managers/:stream_name renders the current state and the state rebuilt step by step (newest step first) — each row shows the event, its timestamp and the state after applying it
• the extension serves its own stylesheet, built with Tailwind from the gem's views only (make css), so the browser's asset pipeline stays untouched

Design notes:

• Recognition via registry, not constant resolution: including ProcessManager.with_state registers the class; parse_stream_name is a hash lookup. No Object.const_get on URL-derived input, no autoloading side effects, and hand-rolled classes that skip with_state are deliberately not browsable.
• Replay through a public API: ProcessMethods#replay(events) folds apply from initial_state and returns the successive states. Production build_state uses the same fold, so the view shows exactly what the process computed. act is never invoked — browsing is side-effect free.
• StateReplay returns steps plus the current state; the route handler is plain plumbing.

Example wiring in a Rails app:

require "ruby_event_store/process_manager/browser_extension"

mount RubyEventStore::Browser::App.for(
  event_store_locator: -> { Rails.configuration.event_store },
  extensions: [RubyEventStore::ProcessManager::BrowserExtension.new],
) => "/res",
as: :ruby_event_store_browser_app