web: embed-followups - #871

work#361: per-copy key targeting, embed-scoped fullscreen, one shared wasm compile, two-copy test (web/tests/embed.test.mjs), run inside the existing test-web-text-input.sh step (round-2 must-fix verified). Clean at 095720e from macos/agy, macos/claude, windows/codex (no findings; its integration failure fails on main too). Full CI green. foxnne's yes for the ci.yml/AGENTS.md lines: card p871-ci. Credit to Michael Belousov (#835, #829) added to the body. Minors macos/claude F1 (fullscreen element stack in the fake) and F2 (text-input tests in CI) are follow-ups.