feat(fpv-drone): add the FPV drone explainer

[veelenga]

Overview

• add the fpv-drone explainer: one 80 second reconnaissance sortie of a 7.5-inch long-range quad, from takeoff beside a dugout to the landing, told in six chapters (overview, the motor mix, the flight controller, the two radio links, the battery and payload, the limits) with a field diorama, a detailed drone model, link beams, the flown track and spin arrows
• model from a sourced facts sheet: thrust and tilt, the motor mix and spin directions, flight modes, packet rate against range, analogue and digital video, the battery, the weight spiral and the speeds, with the scope kept to how a quadcopter flies and is controlled
• air effects: downwash under each propeller scaled by its thrust share, a dust wash at takeoff and landing, a sky gradient with clouds and wind streaks past the camera at cruise
• goggles view in the last chapter: the stage is framed as a visor with a caption and scanlines while the camera rides in the drone's nose, the copy says so, and the frame drops as soon as the reader pulls the camera out
• let a follow view turn with its anchor in @core/scene/camera: follow takes 'position', 'heading' or 'attitude', the tween turns with it, and every explainer's follow: true becomes 'position' with no change in behaviour
• let an explainer set the camera target's floor margin, so a small model resting on the ground is not pushed down the frame
• copy in all eight languages, each with a native-reader pass from the English meaning, with zero dashes and descriptions within the limits
• cover and link preview image, chapter widgets (moment, move, flight mode, packet rate, video and speed chips, tilt and payload sliders), gauge readouts for the clock, speed, height, battery and distance

Notes

Browser QA: the six chapters walked on desktop 1440 × 900 and iPhone 13 in English, Ukrainian and German with no page or console errors, no untranslated keys, no labels over the dock, readouts that follow the scrubber and no overflow. Lint, the full test suite (3,052 tests) and the build with the site check pass.

One report from review is still open: a blink in the controller chapter's close view on a real GPU, which did not reproduce in headless Chromium.