Persist Scenario Studio authoring state

[caoergou]

Summary

• persist Scenario Studio layout preferences, including inspector width, canvas mode, and viewport preset
• preserve the active authoring task, subtool, selected graph node, and selected rehearsal beat across reloads
• validate stored state, isolate it by authored input, and safely fall back for corrupt, stale, or incompatible values
• make the inspector splitter keyboard-adjustable with current ARIA values

Why

Scenario Studio already preserved selection during a valid hot reload in memory, but a browser refresh discarded the writer's layout and authoring context. This completes the remaining persistence item in #286 without changing ContentPack, snapshot, or embedded-engine state.

Validation

• npx vitest run test/scenarioTools.test.ts
• npm test -- --run
• npm run typecheck
• npm run typecheck --workspace @caoergou/xp-scenario-tools
• changed-file ESLint and Prettier checks
• npm run guard:nocheck
• npm run guard:purity
• npm run scenario:build
• development and packaged Scenario Studio virtual modules compiled through Vite

Closes #286