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.

CLAUDE.md

@@ -8,8 +8,9 @@ See @AGENTS.md for project overview, architecture rules, and the AI developer pr
 - **[docs/spec/](docs/spec/)** — authoritative behavioral specification (sections 00–10):
   the **what**. This is the contract; implement to it.
 - **[docs/architecture/](docs/architecture/00-overview.md)** — architecture playbook
-  (00–08): the **how** (state, persistence, modals, routing, rendering, inference,
-  relationships, vega-editor techniques). Self-contained — no external repo needed.
+  (00–09): the **how** (state, persistence, modals, routing, rendering, inference,
+  relationships, vega-editor techniques, visual design). Self-contained — no external
+  repo needed.
 - **[docs/IMPLEMENTATION-PLAN.md](docs/IMPLEMENTATION-PLAN.md)** — incremental milestone
   plan (M0–M6), MVP boundary, per-milestone tests + manual checks, and an architecture
   reference index.