Use live support data for BrowserStack

[adriaandotcom]

What changed

• build the BrowserStack matrix from the live BrowserStack catalog, endoflife.date, MDN browser metadata, and Can I Use global usage
• apply one policy to every browser, including Internet Explorer: vendor support, two years after EOL, or more than 0.1% global usage
• select oldest and newest eligible browser, OS, and mobile-device boundaries instead of every available combination
• retry and validate all live sources, then fail closed if support data or required coverage is missing
• lower the minimum matrix size from 20 to 10 while requiring Chrome, Edge, Firefox, Safari, Windows, macOS, Android, and iOS
• run BrowserStack only for pull requests and document the customer-facing support policy

The live matrix currently contains 17 sessions instead of 63. IE11 remains because its current global usage is above the threshold; IE9 and IE10 fall out through the same policy.

Validation

• npm run test:matrix — 11 passing
• npm run build -- testing
• Prettier check and git diff --check
• live BrowserStack/source selection — 17 sessions
• targeted BrowserStack IE11 run on Node 16.16 — 2 passing

[github-actions]

Claude review checkpoint

Reviewed commit 97aacc3f3642f520327b74b80f04fd5463a0eaf9. This is used to keep later automated reviews focused on changes Claude has not checked yet.