docs(soma): add the "compose existing components, flag gaps" rule

New component-authoring rule, per request: when building a component (or its
demo, or any UI), compose the framework's existing soma/eidos components rather
than re-implementing primitives inline. If a needed building block does not exist
as a framework component, flag the gap so it gets built as a proper reusable
component (its own morfo + soma + eidos) and then composed — never an ad-hoc
reinvention that drifts from the system. The picker family (A27) is the canonical
example of composing existing components.

Full rule in COMPONENT_GUIDE "Before You Start" §4; one-line pointer added to
CLAUDE.md Key Conventions so every session loads it.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -112,6 +112,11 @@ this to detect drift between the morfo contract and the eidos rules.
 - **Read before acting.** When told to read a file, read it. Don't paraphrase.
 - **Verify before reporting done.** Run `npm run check` and the relevant
   vitest scope. UI claims need a browser check; if you can't run a browser, say so.
+- **Compose existing components; flag gaps.** When building a component (or its
+  demo, or any UI), compose the framework's existing soma/eidos components
+  (`Button`, `Field`, `Popover`, `Icon`, …) — never re-implement a primitive
+  inline. If a needed component is missing, **flag the gap** so it gets built as
+  a reusable component, never reinvented ad-hoc. Detail: `soma/COMPONENT_GUIDE.md` §4.
 - **No backward-compat shims** when relocating code. Update consumers and delete
   the old path; don't leave a re-export.
 - **No re-export façades** between layers (e.g. soma must not re-export dias —

src/uix/soma/COMPONENT_GUIDE.md

@@ -61,6 +61,23 @@ Accepted exceptions (composition criterion waived when WAI-ARIA defines a tight
 - `Announce` — a live-region primitive per WAI-ARIA 1.2 live regions; meets complex-behavior via dual-region A/B dispatch + auto-clear + priority routing, even though its surface is a single region per priority.
 - `Progress` / `Meter` — canonical single-element roles with computed ARIA values and CSS custom properties for the decorative indicator; shipped with an `Indicator` part so consumers have two slots (the role host and the fill), crossing the composition threshold.
 
+### 4. Compose existing components; flag gaps
+
+**Dogfood the framework.** When a new component — or its demo, or any UI you
+build — needs a building block the framework already provides (`Button`, `Field`,
+`Popover`, `Dialog`, `Icon`, `Calendar`, `Select`, …), **compose the existing
+soma/eidos component**. Never re-implement a primitive inline or hand-roll a
+one-off. The picker family is the canonical example: pickers compose `Popover` +
+`Field` + `Calendar`/`Slider` with shared state instead of reinventing any of
+them (A27).
+
+If a needed building block **does not exist** as a framework component, do **not**
+silently inline a bespoke version. **Flag the gap** — report that component `X`
+is missing — so it can be built as a proper, reusable component (its own morfo +
+soma + eidos) and then composed. A missing component is a signal to create it (or
+record the need), never an excuse for an ad-hoc reinvention that drifts from the
+system.
+
 ## File Structure
 
 ```