feat: rewrite nova_liveboard on Nova + Datastar (drop Arizona)

[Taure]

Why

Arizona is deprecated for this use: it moved its HTTP layer from Cowboy to its own roadrunner server, and arizona_nova never followed, so the Nova(cowboy) + arizona_nova bridge the dashboard relied on is dead. This rebuilds nova_liveboard on Nova + Datastar - server-rendered HTML, live updates over SSE, no Arizona, no JS build step, no off-origin requests. Same proven pattern as gakudan_liveboard.

What changed

• Kept the BEAM data layer (nova_liveboard_data) intact.
• Replaced every Arizona view with a pure iodata() render layer (nova_liveboard_html) in a new "BEAM mission control" design - deep-space instrument panel, a live vitals deck, nova-gold + signal-cyan accents, self-hosted IBM Plex Mono + datastar.js under a strict default-src 'self' CSP.
• Live transport (nova_liveboard_sse): one long-lived SSE stream per page region via a registered {stream, ...} return-handler (as a fun/3); polled regions repaint on a timer, the requests feed is event-driven. No Nova patch needed (see Support streamed / long-lived responses (SSE, server-push) — controllers can only return a single buffered reply nova#387).
• New Requests page + request tracer: a nova_plugin records per-request timing / reductions / handler MFA for every route on the node, plus an opt-in deep mode that scope-traces a request (erlang:trace with set_on_spawn) and shows the exact tree of processes it spawned. Overhead is only paid while armed.
• Preserves every existing page (overview, processes, metrics, ETS, applications, ports, supervisors, and the Kura database/schemas pages).

Pages

Overview · Metrics · Processes · Requests (new) · Supervisors · Applications · ETS · Ports · Database/Schemas (Kura).

Enabling request tracing

Register the plugin globally in the host's Nova config:

{nova, [{plugins, [
    {pre_request,  nova_liveboard_trace_plugin, #{}},
    {post_request, nova_liveboard_trace_plugin, #{}}
]}]}.

Verification

• rebar3 fmt --check, rebar3 xref, rebar3 dialyzer all clean.
• rebar3 eunit - 49 tests, 0 failures (data, html render + escaping, tracer ring buffer + synthetic spawn-tree, plugin summary build, SSE frame shape).
• Final in-browser verification happens when embedded in a host Nova app (this is a library; no standalone harness was added).

Breaking

Drops the arizona_core / arizona_nova dependencies. Mount via {nova_apps, [nova_liveboard]} instead of arizona_nova, and register nova_liveboard_trace_plugin to enable the Requests page.

Follow-ups

• Add a Taure/erlang-ci workflow (repo currently has none).
• Push the latent datastar_nova fun/3 handler fix upstream.