feat(player-ui): add pre-reader controls and onboarding

docs/PRE_READER_USABILITY.md

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+# Pre-reader usability observation guide
+
+## Purpose
+
+Use this short, consent-based observation to verify the player interface with
+a child who does not rely on reading. It is an acceptance activity for
+Milestone 015, not a source of analytics. Do not record names, images, audio,
+contact details, or any other identifying information.
+
+## Setup
+
+- Use the player screen at a `412 x 916` CSS-pixel viewport with sound enabled.
+- Start a normal two-player match with the observer screen hidden.
+- An adult may read a prompt once but must not point to the answer or operate
+  the child's device.
+
+## Prompts
+
+Ask the child to demonstrate each task. Mark it as completed only when the
+child succeeds without reading the Russian support text.
+
+| Task | Expected non-reading cue |
+| --- | --- |
+| Move the hero | circular joystick and character movement |
+| Find and pick up a key | key shape and pickup feedback |
+| Recognize farther vision | eye symbol and map expansion |
+| Follow the compass | arrow direction and compass icon |
+| Find the exit | outlined exit shape and owner character shape |
+| Recognize a wall | wall shape plus rejection feedback |
+| Recognize the result | result icon/overlay plus local feedback |
+
+## Record
+
+Record only the date, viewport, browser, whether Russian labels were read,
+and the seven task outcomes. Summarize unclear cues and any adult assistance.
+If no representative child observation is feasible, the project owner may
+approve a documented deviation instead; include the reason and the remaining
+risk in the active milestone evidence.