feat(experiments): WS5b participant screens in built-in and custom studies

[jdpigeon]

Implements docs/superpowers/plans/2026-09-24-ws5b-participant-screens.md: the approved #273 participant screens now run in the four built-ins and Custom. Imported jsPsych/lab.js studies are not touched (deferred, see TODOS).

Commits

• Task 1: built-ins get a screens.ts each; Instruction / transition / End content → builders; the obsolete intro copy is deleted; stories import the real sources; key-contract test.
• Task 3: isEEGEnabled runtime prop → parameters.isEEGEnabled, so the stillness line appears only on EEG runs (tested with real lab.js).
• Task 2: Custom builds its screens in before:prepare from the teacher's conditions; the intro stays a ${this.parameters.intro} placeholder.
• Task 4: CSS fix so Preview's non-fullscreen mount contains the screen (before this, the footer spilled over Stop preview), plus docs.

Verification

• npm run typecheck 0 errors · npm run lint 0 errors (155 warnings, all pre-existing) · npm test 57 files / 297 passed, 1 skipped · node tests/electron-smoke.mjs PASS.
• Electron playtest with the Fixture headset (screenshots are local to the agent, in /tmp/ws5b-screens/). Faces/Houses on EEG: instructions with the stillness line, practice showing Practice trial N of 6, the transition screen with the same keys, main trials showing Trial N of 120. Behavior-only: no stillness line anywhere; a full run reached the End screen and then Run complete. Preview on Design and on Collect: the new screens appear with no stillness line.

Found, not fixed (pre-existing): Q never skips practice in lab.js 23 (details in TODOS). Stroop, Search, Multitasking, Custom and the imported studies were not exercised in the Electron playtest. Their keys are covered by unit tests.