Phone on its side: STOP fixed top-right inside the safe area, sticks in the lower corners once a turn starts, proven by a stylesheet test #312

Replaces #93 (piece of #49). It was parked twice, but pulls #301 and #306 carried the same commit, 46e5866 (refs/pull/301/head on origin). Its suite passes apart from one sandbox-only install miss, so the work is sound and the preview deploy is what failed. The old criteria also asked for a Playwright test and screenshots. The repository has no browser dependency, so those could never be met, and they are dropped here.

Start from 46e5866: cherry-pick it onto main, or redo the same small change in Software/so101/src/viewer.html.

Done, under (orientation: landscape) and (max-height: 500px) (844x390, 915x412):

• [ ] The video fills 100dvh. STOP is position: fixed top-right inside the safe-area insets, with a fixed width, so "Stop" and "Stopped" sit in the same place.
• [ ] No backdrop-filter, transform or filter on an ancestor pins STOP to it.
• [ ] A turn that starts with the phone already on its side tucks the sheet away, so the joint sticks show in the lower corners.
• [ ] Portrait and desktop are unchanged. There is still exactly one [data-stop], and src/stop-layout.test.ts, src/stop.test.ts and src/portrait-layout.test.ts pass.
• [ ] src/landscape-layout.test.ts reads the served stylesheet and checks the points above. It fails on main and passes with the change.
• [ ] The preview deploys and / answers on it. If the deploy fails again, the pull quotes the deploy log's error and fixes that cause, without widening the scope. No browser dependency is added.

Files: Software/so101/src/viewer.html, Software/so101/src/landscape-layout.test.ts, Software/README.md (the landscape note).