feat(docs): overhaul guide experience

[Exoridus]

Problem

The guide infrastructure (typecheck gate, source-backed snippets) is reliable, but onboarding and navigation still require too much prior knowledge. Chapters had no Previous/Next, no learning metadata, no visible Guide → Playground → API path, the landing page was a flat chapter list, titles/numbers were duplicated across the structure and frontmatter (and had drifted), and one chapter was orphaned and unreachable.

Solution

A focused redesign of the guide experience on top of that infrastructure:

• Information architecture — site/src/lib/guide-structure.ts is now the single source of truth for ordering, grouping, and learning metadata (level, learning goals, prerequisites, playground examples, API links). Chapter numbers are positional, so reordering never means renumbering by hand. MDX frontmatter is slimmed to { title, description }. Part renames: Getting Started, Debugging & Performance, Reference & Deployment. The orphaned audio-reactive-visualization chapter is reattached under Audio. New introduction/project-structure chapter.
• Landing / Start here — hero with the create-exo-app command and primary actions, a numbered recommended learning path (What is ExoJS → Setup → First scene → Frame loop → Input → Audio → Orb Dodge → Deploy), topic entry cards, and a Guide → Playground → API orientation strip.
• Page UX — chapter header with a level badge, auto-computed read time, "What you'll learn", and "Before you start"; a structural Previous/Next pager at the end of every chapter; the redundant in-content H1 is hidden.
• Reusable components — Callout (note / tip / important / common-mistake / webgpu / browser — label + glyph, never colour alone), TryIt (compact Playground + API bridge, slugs validated at build time), NextStep, GuideMeta, GuidePager.
• Critical onboarding rewrites — What is ExoJS?, Setup (create-exo-app first), Project structure (source-backed from the minimal template), Your first scene, Scene lifecycle, Keyboard, Audio basics, Debug layer, Performance, Build Orb Dodge.
• Guide → Playground → API flow — TryIt added to Your first scene, Scenes, Scene lifecycle, Graphics, Sprites, Keyboard, Gamepad, Audio basics, Debug layer, Performance, and Orb Dodge.
• Source-backed snippets — minimal template main.ts / MainScene.ts get guide: regions, consumed by the new Project structure chapter.

Scope

Docs / site / DX only. No engine features, no runtime API changes, no new packages, no versioning, no release notes.

Validation

Check	Result
pnpm typecheck	✅
pnpm typecheck:guides	✅ 27 extracted, 22 no-check, 217 partial
pnpm typecheck:examples	✅
pnpm lint:strict	✅
pnpm test	✅ 141 files, 1840 tests
new test/site/guide-structure.test.ts	✅ 19 tests (structure↔content reconciliation, metadata + cross-ref validation, prev/next linearity, landing refs)
pnpm build + pnpm verify:exports	✅
pnpm verify:create-exo-app	✅ 45/0
pnpm site:build	✅ 572 pages
pnpm test:examples:smoke	✅ 118/0

pnpm format:check reports 6 pre-existing offenders unrelated to this PR (rollup.config.ts, scripts/extract-guide-snippets.ts, scripts/verify-create-exo-app.ts, test/site/example-search.test.ts, test/site/source-snippets.test.ts, tsconfig.guides.json — all unchanged here); every file touched by this PR is prettier-clean.

Visually reviewed (desktop 1440×1000 + mobile 390×844, dark + light): guide landing, What is ExoJS?, Setup, Project structure, Your first scene, Keyboard, Audio basics, Debug layer, Performance, Build Orb Dodge, Troubleshooting, Deployment.

Release note

Release preparation (PR #93) remains postponed and must be re-created or updated after this overhaul lands. This PR contains no version or changelog changes.