[maxheld83]

Improves the Shiny dev server served by shiny / pc-up-r-shiny.

• Browser: runApp(launch.browser = <fn>) calls utils::browseURL() with the actual URL and writes it to $NULLKOMMA_PROCESSES/shiny/url. Shiny's Listening on … line shows the URL in the process log. The task sets a default R_BROWSER (open/xdg-open), because Nixpkgs' R leaves it empty. nix run .#shiny does not open a browser.
• Reload: the existing autoreload is kept. The watcher package is added, as Shiny recommends for autoreload.
• Alive indicator: a process-compose readiness_probe (exec + curl against the recorded URL) shows Ready/NotReady in the TUI. A very high failure_threshold makes the probe report status only; process-compose stops a process when the threshold is reached.
• Fixture: tests/fixtures/r-shiny-hello contains Shiny's Old Faithful example. The dev task try-shiny serves it from a scratch copy with pc-up-r-shiny so you can try it by hand.
• Tests: the r-shiny check now asserts the browser launch, the readiness probe passing and failing, and reload after an edit. A unit test evaluates the hello fixture.

tasks all and nix flake check pass on aarch64-darwin.