fix: resolve nova_apps in declaration order and let applications override the default error pages

[Taure]

Two defects found while boot-testing Nova end to end for #405. Both are on master today and neither depends on the routing rewrite, so they are split out here to be reviewed — and backported — on their own.

resolve_nova_apps/2 returns applications in the wrong order

resolve_nova_apps/2 (added in #380) calls lists:reverse/1 in its base clause, on an accumulator that the caller then keeps accumulating into. So the reversal happens once per recursion step rather than once at the end.

The effect is not limited to nested applications, which is what I first assumed — it is wrong for any nova_apps list of two or more entries:

resolve_nova_apps([a, b], [])     %% => [b, a]
resolve_nova_apps([a, b, c], [])  %% => [c, a, b]

Routes are first-wins, so this silently inverted route precedence between sibling applications. Anyone relying on declaration order to decide which application owns an overlapping path has had the opposite of what they asked for.

It also resolves nested applications with the wrong key:

Nested = application:get_env(App, nova_apps, [])

App here can be a {Name, Options} tuple — the form guides/multi-app.md documents for setting a prefix — and application:get_env/3 misses on a tuple key and returns the default. A sub-application declared in tuple form therefore never has its own nova_apps resolved at all.

Rewritten with the seen-set kept separate from the output, so the reversal happens once, and with the application name extracted from either form before the environment lookup.

An application's own status-code routes are unreachable

start_cowboy/1 compiled [nova, App | ExtraApps], so Nova's own routes went in first. Routes are first-wins, so Nova's default 404 and 500 always won and this in an application's router:

routes(_Env) ->
    [#{routes => [{404, fun my_controller:not_found/1, #{}}]}].

compiled, registered, and then never matched. No error, no warning — the custom error page simply never appeared.

nova is now compiled last, which makes its error pages the defaults they were always meant to be. An application without its own status routes is unaffected.

The ordering decision is extracted into compile_order/1 so it can be tested without starting a listener.

Tests

test/nova_sup_tests.erl covers both. 8 of its 14 cases fail against the current implementation — flat ordering, nested ordering, cycle handling, deduplication, both tuple-form defects, and the compile order.

rebar3 eunit 372/372, xref and dialyzer clean.

Relationship to #405

#405 contains both fixes already, as part of its nova_sup rewrite. If this merges first, #405 needs a git merge origin/master and its own nova_sup kept — it is a strict superset. Happy to land them in either order; this one is the smaller, safer half.