Add visual design language (IBM/Carbon-inspired) and schedule it as M1.5

Defines the visual contract before the design-application work, so M2+ build on settled tokens.

- docs/architecture/09-visual-design.md: principles + deliberate divergences (square chrome, free color/theming), token system (IBM Plex type, 8px spacing, role-based color, motion), component conventions, and a source-repo map for extending the research
- docs/architecture/visual-specimen.html: standalone kitchen-sink specimen with a live theme x accent switcher; doubles as the tokens.css sandbox
- IMPLEMENTATION-PLAN: new M1.5 'Visual design foundation' milestone, plus ground-rule/cross-cutting/reference wiring
- index links: 00-overview, CLAUDE.md, AGENTS.md (architecture playbook now 00-09)

Applying the design to the M1 surfaces is deferred to a separate session.

docs/architecture/00-overview.md

@@ -29,6 +29,7 @@
 | 06 | [Type Inference & Profiling](06-type-inference.md) | Pure, portable column-type inference (number/text/date/boolean) and the dataset profile shape. |
 | 07 | [Naming & Relationships](07-naming-and-relationships.md) | Unique-name enforcement + import auto-suffix; the bidirectional snippet↔dataset name link; rename propagation into specs. |
 | 08 | [vega/editor Techniques](08-vega-editor-techniques.md) | Reference brief: borrowable Monaco-schema wiring, vega-embed lifecycle, two-tier validation, and data-flow/debounce techniques distilled from the official Vega-Lite editor — plus where we do better. |
+| 09 | [Visual Design Language](09-visual-design.md) | The *visual* contract: principles inspired by IBM/Carbon, deliberate divergences (square chrome, free color/theming), the token system (Plex type, 8px spacing, role-based color, motion), component conventions, and where to mine the Carbon/IBM source repos for more. Companion: [`visual-specimen.html`](visual-specimen.html). |
 
 ## The non-negotiable layering (every doc assumes this)