feat(blackjack): make the blackjack game playable end-to-end

[netqo]

Summary

Wires classic blackjack (CU-06) end-to-end - the fifth and final game, completing the games card. Deal, then Hit / Stand / Double against a dealer that draws to 17, with the opening cards and the dealer's draws animated one at a time.

Rationale

Same round pattern as the other games, with the most hand logic:

• BlackjackMath is a pure, unit-tested scorer over the engine's card indices (soft-ace totals, bust, natural blackjack, dealer-hits rule).
• GameRepository.startBlackjack validates the stake (no debit) and draws the shuffled deck on the C++/JNI engine (committed via the hash). settleBlackjack applies the net in one write and records the round with the player/dealer totals - settling net at the end is what makes Double a single doubled settle rather than a second debit.
• BlackjackViewModel owns the hands and deals card by card with a delay (player/dealer alternating, hole card hidden); the dealer reveals and draws with delays too. Hit / Stand / Double; Split is intentionally out of scope.
• RoundDetailViewModel branches by game to show "Your Hand" vs "Dealer" with the real seeds.
• Phases (Idle / Dealing / PlayerTurn / DealerTurn / Result) keep the action grid in place (disabled while dealing) so the layout stays stable.

Verification

• ./gradlew testDebugUnitTest detekt ktlintCheck assembleDebug compileDebugAndroidTestKotlin green locally (JDK 17).
• New tests: BlackjackMathTest, BlackjackViewModelTest (stand win, hit bust, natural blackjack 3:2, invalid bet), startBlackjack/settleBlackjack cases in GameRepositoryImplTest.
• Installed the debug APK on a physical device: dealt hands (cards animate in), hit / stood / doubled, busted, hit naturals; the dealer reveals and draws to 17; balance settles net; rounds show in History and Round Detail (hands + real seeds) and in the Profile P&L.

Test plan

• Deal lays the opening cards out one at a time; the dealer hole card is hidden.
• Hit draws; busting over 21 loses.
• Stand plays the dealer (draws to 17) and compares.
• Double doubles the stake, draws one card and stands.
• Natural blackjack pays 3:2; pushes return the stake.
• The round lands in History / Round Detail (hands + real seeds) and the Profile P&L.

Related issues

Refs the games-card work for the final entrega (Blackjack is the 5th of 5 games; the games card is now complete).

Checklist

• Commit messages follow Conventional Commits (see CONTRIBUTING.md).
• Branch name follows the naming convention (feat/, fix/, chore/, etc.).
• The change is scoped: no unrelated edits sneaked into this PR.
• If user-visible behavior changed, CHANGELOG.md has been updated under [Unreleased].
• If a new dependency was added, it is justified in the PR description. (No new dependencies.)