docs: Update documentation for screenReaderSpeed setting (PR #33)

[Copilot]

PR #33 introduced a new screenReaderSpeed accessibility setting (uint8_t, persisted under key "screenReaderSpeed", fires OnScreenReaderSpeedChanged) with no corresponding documentation updates.

Changes

• PRODUCT.md — Added screen reader speed to Core Features and Accessibility-Focused Applications use case
• ARCHITECTURE.md — Updated settings count (18 → 19); added screenReaderSpeed entry with type, storage key, and notification event; enumerated representative notification events in the Notification System section
• plugin/CHANGELOG.md — Added v2.4.0 entry for the new setting

SME Review Needed

• Valid range — uint8_t (0–255) is documented as-is; semantics (units, valid range, default) should be confirmed once SetScreenReaderSpeed/GetScreenReaderSpeed implementation lands
• Version — Confirm v2.4.0 aligns with project versioning policy

Warning

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• https://api.github.com/graphql
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