feat(design-system): document BlockUI logic and grammar

[mahanti]

Adds System → Logic & grammar at /design/grammar, a compact reference for selecting BlockUI tokens and composing supported interfaces. Fifteen always-visible tables cover role resolution, all 19 typography roles, color/emphasis, interaction states, geometry, component contracts, intent and modes, applet slots, visualization selection, palettes, the 12 composition defaults, size/data fallbacks, and generated-response constraints.

Each topic pairs usage with its boundaries and links to a verified upstream snapshot. The page identifies source-specific exceptions, provisional guidance, and differences from Buzz’s active type, font, and density contracts. It summarizes the reference without importing private fonts, assets, implementations, or a runtime service. Source links require Block access. It does not change token values or implement the upstream planner.

Every topic renders as a heading and a flat table, with no collapsible sections. Topic links support keyboard navigation and refresh. Tables scroll horizontally on narrow screens and remain keyboard-accessible. Theme and Normal/Compact density remain shared.

Stack 7: #7522 → #7523 → #7525 → #7533 → #7535 → #7537 → this PR. Base: codex/design-system-component-pages. Keep draft.

Validation:

• Production build, TypeScript, formatting, typography, contrast, token freshness, and all 8 token/schema/renderer checks pass.
• All 35 Chromium tests passed in the full run. The two grammar tests passed again after the flat-table refinement, including always-visible content, hash navigation, phone-width scrolling accessibility, and both themes and densities.
• Manual local inspection at desktop and phone widths; automated table visibility, navigation, direct-link, responsive, and light/dark accessibility coverage.
• Repository-wide just ci passed the earlier checks and desktop tests, then stopped at the unchanged buzz-terminal::lifecycle_tests::default_prog_child_observes_the_login_argv0: the child did not report $0 within 10 seconds. The same failure occurred before this change. Later repository-wide stages were not reached; no terminal or Rust code changed.