Add E2E matrix launch smoke foundation

[kieran-osgood-shopify]

Stack Context

This is Phase 1 of the BrowserStack Maestro E2E stack for Checkout Kit.

Parent issue: https://github.com/shop/issues-checkout-kit/issues/1095
Epic: https://github.com/shop/issues-checkout-kit/issues/1084

What?

Adds the shared E2E foundation:

• shared Maestro launch smoke at e2e/tests/shared/launch-smoke.yaml
• initial matrix config at e2e/config/matrix.yml
• matrix validation/expansion CLI at e2e/scripts/e2e_matrix
• tests for matrix behavior
• updated E2E README with local run instructions and shared app contract

Why?

The E2E pipeline should be matrix-driven from the start so later phases can fan out across app targets, OS tracks, and suites without redesigning the Bitrise pipeline.

This PR intentionally avoids Bitrise and BrowserStack execution. It establishes the minimum shared smoke suite and the matrix contract used by later PRs in the stack.

Validation

shadowenv exec -- ruby e2e/test/e2e_matrix_test.rb
shadowenv exec -- ruby e2e/scripts/e2e_matrix validate
shadowenv exec -- ruby e2e/scripts/e2e_matrix expand --index 0

[kieran-osgood-shopify]

• Soft-launch E2E PR checks #364
• Report E2E results to GitHub #363
• Run BrowserStack Maestro from E2E matrix rows #362
• Build React Native iOS E2E IPA artifact #361
• Build React Native Android E2E APK artifact #360
• Add Bitrise E2E pipeline skeleton #357
• Add E2E matrix launch smoke foundation #356 👈 (View in Graphite)
• main

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