Redesign data-load state model and normalize the Apollo cache

[solcott]

Summary

Reworks how the app models the result of a data load and configures Apollo's
normalized cache so domain entities are shared across queries. Also bumps AGP.

The state redesign makes Data/Error total and non-nullable, replaces a
boolean-soup view state that allowed illegal combinations with a single generic
holder, and introduces a transport-agnostic error vocabulary so the model isn't
coupled to Apollo. The cache change is the plumbing that will let a future write
to one entity propagate to every screen that displays it.

Changes

model — transport-agnostic result type

• Replace Response<T> (Loading/Data/Error) with Outcome<T>: a
  Data(data, origin) / Error(cause, origin) result. Loading is no longer a
  result case — it's an in-flight concern the consumer tracks.
• Add DataError, a client-neutral failure vocabulary (Network, Http,
  Api, Serialization, Unknown) so any source (Apollo, Ktor, Store) maps its
  own errors into a shared domain type.
• Add Origin (Cache/Network), recorded on every Outcome.

repository — Apollo → Outcome adapter

• mapToOutcome tags each emission's Origin from isFromCache, categorizes
  ApolloExceptions into DataError, and drops cache-miss emissions instead of
  surfacing them as errors.
• Depend on apollo-normalized-cache to read per-response cache metadata.
• Remove the dead ContinentRepositoryImpl.kt.

presenter — one generic view-state

• Replace LoadState/CountriesState/ContinentsState with a single generic
  ContentState<T> plus a LoadStatus (Idle/Loading/Failed) sum
  type, eliminating illegal states.
• Derive Loading from the flow lifecycle (onEach/onCompletion) with no
  fetch-policy assumption; re-assert Loading on refilter via collectLatest
  while keeping the current list visible (stale-while-revalidate).
• Move CountryDetailScreen.State onto ContentState<CountryDetail?> with a
  derived isNotFound.

ui

• Resolve DataError to localized strings; show a refresh indicator while
  reloading over already-visible data.

network — normalized cache configuration

• Add @typePolicy(keyFields: "code") for Country, Continent, and Language
  so every query shares one record per entity instead of embedded copies.
• Select continent { code } in the Countries/CountryDetail queries, which
  normalization by key requires.

Build

• Update Android Gradle Plugin to 9.3.1.

Notes

• Generated Apollo code is build output and is not committed.
• The cache normalization has no visible effect yet — the flows still use
  one-shot .toFlow(), not .watch(). This is the groundwork for cache-driven
  reactivity (a follow-up would add .watch() + local cache writes).

Testing

• ./gradlew assembleDebug test ktfmtCheck — all green.