docs(testing): document the Tier 0–4 BitBox test strategy

[TaprootFreak]

Closes Acceptance Criterion #6 of #314 — "Documentation: `docs/testing.md` explains when to use which tier, with examples."

Summary

A single page that captures everything we've learned across Phase 0 (#319) and Phase 1 Foundation (#320 / #321) about how to test BitBox-touching code without re-deriving the rules every PR:

• Tier matrix (0–4) with hardware / CI status for each, so reviewers can tell at a glance what a test does and doesn't prove
• Decision tree for picking a tier — "if a Tier 0 test would have to mock the very thing under test, drop down a tier"
• Tier 0 patterns with concrete excerpts from real test files:
  • Cubit / Bloc tests (`bloc_test` + `mocktail.Mock`) pointing at `test/screens/kyc/cubits/kyc/kyc_cubit_test.dart`
  • Widget tests (`pumpApp` + `MockCubit`) pointing at `test/screens/kyc/steps/kyc_email_page_test.dart`
  • Service + HTTP tests (`http/testing` `MockClient` + `_MockAppStore`) pointing at `test/packages/service/dfx/dfx_bank_account_service_test.dart`, including the `setAuthToken('test-jwt')` pre-seed trick for DFXAuthService-derived services (the gotcha that broke CI on fix: keep dashboard buy actions visible #321)
• Tier 1 patterns for `FakeBitboxCredentials` with the `FakeBitboxBehavior` matrix and the disconnect-flip-to-success reconnect pattern, pointing at `test/integration/kyc_sign_flow_test.dart`
• Tiers 2–4 marked deferred with status pointers back to the corresponding phase of Comprehensive BitBox testing infrastructure (Tier 0–4) #314
• Mocktail gotchas (`Future` + `thenAnswer`, `registerFallbackValue`, private mocks)
• Add-tests checklist for PRs touching `KycCubit` / `Eip712Signer` / `DFXAuthService` / `BitboxCredentials` / `bitbox_flutter`

CONTRIBUTING.md's Testing section gets a one-line cross-link.

Test plan

• Doc compiles / renders (no broken intra-repo links in the markdown)
• Every code example matches a real file already on `develop`
• `flutter analyze` — no new issues (4 pre-existing errors in `test/screens/home/home_bloc_test.dart` are unrelated to this PR; verified they exist on `develop` too)