feat(docs): scaffold docs workspace and validation

[Mekussdev]

Summary

• Adds the initial apps/docs TanStack Start workspace on Vite 7 with a placeholder SO4 Docs route and dev server on port 3001.
• Adds docs keyboard navigation shortcuts for search, sidebar focus, previous/next page navigation, and a discoverable shortcut list using KeyboardShortcut labels.
• Adds check:content with frontmatter, nav manifest, slug stability, updated freshness, filename, orphaned asset, and duplicate-title validation plus fix mode for dates and manifest ordering.
• Adds representative docs accessibility tests for page structures, interactive states, heading order, landmarks, themes, search, and tabs.
• Wires docs content validation into CI and contributor docs, and fixes the GLV router client to use the typed NETWORK.rpcUrl config so typecheck passes.

Verification

• bun run --cwd apps/docs dev served http://localhost:3001/ and local curl returned 200 6586.
• bun lint passed with existing warnings in apps/web and packages/ui.
• bun typecheck passed.
• bun run check:tokens passed: scanned 286 files, 0 violations.
• bun run check:content passed: 0 errors.
• bun run test passed; docs suite: 3 files, 23 tests, runtime about 8.34s locally.
• bun run test:coverage passed; docs coverage suite runtime about 11.00s locally.
• bun run build passed and apps/docs emitted .output/.
• Integration checks passed: apps/s03-indexer codegen/build/test, sync:contracts:local, manifest validation, apps/web typecheck, apps/web build.
• Clean-room verification passed in /tmp/so4-cleanroom-mg8zmo/repo: bun install --frozen-lockfile, then bun lint && bun typecheck && bun run check:content && bun run test && bun run build.

Accessibility and shortcut notes

• Shortcut handler ignores input, textarea, and contenteditable targets; this is explicitly tested.
• Shortcut combinations avoid common browser/screen-reader reserved commands: Mod+K for search follows the product command-search pattern, Mod+Shift+S scopes sidebar focus to docs, Alt+ArrowLeft/Right mirrors browser history-like previous/next expectations, and ? opens only the local shortcut list.
• Axe coverage runs representative docs states in both light and dark themes without disabled rules.

Closes #538
Closes #555
Closes #564
Closes #568

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