feat(eidos): wire variable-font axes (weight range + optical sizing)

`FontFamily.axes` ({ wght, opsz }) was declared but never consumed — the
TYPOGRAPHY_ENGINE_RFC §5 promise was unmaterialized. Wire it:

  - axes.wght -> when a face omits `weight`, @font-face emits the range
    (`font-weight: 100 900`) so one variable face spans the whole axis
  - axes.opsz -> `:root { font-optical-sizing: auto }` so the optical-size
    axis tracks the rendered font-size

Engine-only, inert until a theme declares axes (shipped fonts are static
TTFs — same posture as wide-gamut color: ready, not yet exercised by
assets). Zero change to generated/base.css (no family declares axes).

`npm run check` 0 errors; new test asserts the emission; the generated-css
test confirms base output unchanged.

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

src/uix/eidos/TYPOGRAPHY_ENGINE_RFC.md

@@ -96,8 +96,18 @@ size = clamp(minRem, interceptRem + slope·100vw, maxRem)
 - **`unicode-range`** subsetting (latin / latin-ext separados) → cargar solo lo usado.
 - **`preload`** de la familia crítica (primary regular).
 - **Tipo**: `FontFamily` += `axes?: { wght?: [min,max]; opsz?: [min,max] }` +
-  `sizeAdjust?` / overrides de métricas + `unicodeRanges?`. `source` ya existe.
+  `sizeAdjust?` / overrides de métricas + `unicodeRanges?`. `source` ya existe. ✅
-- **Optical sizing**: `font-optical-sizing: auto` cuando la familia declara `opsz`.
+- **Optical sizing** ✅: `font-optical-sizing: auto` en `:root` cuando alguna familia
+  declara `opsz`.
+- **Eje de peso variable** ✅: cuando un `face` omite `weight` y la familia declara
+  `axes.wght`, el `@font-face` emite el rango (`font-weight: 100 900`) → un solo face
+  cubre todo el eje. (`render-css > renderFontFace`; test en `active-eidos-config.test.ts`.)
+
+> **Estado §5**: el **motor** está cableado — `@font-face` config-driven, fallback
+> anti-CLS, `unicode-range`, y ahora el consumo de `axes` (rango de peso + optical
+> sizing). Inerte hasta que un tema declare `axes` — como el wide-gamut de color: listo,
+> pero no ejercitado por los assets actuales (TTF estáticas). **Pendiente de assets**:
+> migrar a woff2 variable reales; `preload` (cierre aparte).
 
 ## 6. Tracking · leading · features · measure · wrap (Fase 3)
 

src/uix/eidos/active-eidos-config.test.ts

@@ -441,6 +441,26 @@ describe('ActiveEidos config', () => {
 		expect((css.match(/font-family: 'Lora';/g) ?? []).length).toBe(8)
 	})
 
+	it('emits variable-font output from family axes (weight range + optical sizing)', () => {
+		const cfg = structuredClone(THEME_BASE_OPTIONS)
+		const t = cfg.primitives.typography!
+		// Declare `primary` as a variable font: one face spanning the wght axis + an opsz
+		// axis (enables optical sizing). The face omits an explicit weight on purpose so the
+		// range is derived from the axis.
+		;(t.families as Record<string, unknown>).primary = {
+			...t.families.primary,
+			axes: { wght: [100, 900], opsz: [8, 144] },
+			faces: [
+				{ sources: [{ url: '/fonts/variable/InstrumentSans.woff2', format: 'woff2-variations' }] }
+			]
+		}
+		const css = createEidos(cfg).renderStaticCss()
+		// wght axis → @font-face weight range (a single face spans 100–900)
+		expect(css).toContain('font-weight: 100 900;')
+		// opsz axis → optical sizing enabled in :root
+		expect(css).toContain('font-optical-sizing: auto')
+	})
+
 	it('emits the Phase 3 typography scales + optical tracking', () => {
 		const css = createThemeBaseEidos().renderStaticCss()
 		expect(css).toContain('--tracking-tight: -0.02em;')

src/uix/eidos/lib/render-css.ts

@@ -199,6 +199,11 @@ export function renderStaticCss(options: EidosConfig): string {
 
 	if (primitives.typography) {
 		appendTypographyDeclarations(declarations, primitives.typography)
+		// Variable fonts: enable optical sizing globally when any family declares an `opsz`
+		// axis, so the font's optical-size axis tracks the rendered font-size. (RFC §5)
+		if (Object.values(primitives.typography.families).some((f) => f.axes?.opsz)) {
+			declarations.push('font-optical-sizing: auto')
+		}
 	}
 
 	if (primitives.size) {
@@ -1923,22 +1928,31 @@ function renderFontFaceBlocks(typography: TypographyPrimitiveSet): string {
 		if (seen.has(family.family)) continue
 		seen.add(family.family)
 		for (const face of family.faces) {
-			blocks.push(renderFontFace(family.family, face, family.display))
+			blocks.push(renderFontFace(family.family, face, family.display, family.axes))
 		}
 		if (family.fallback) blocks.push(renderFallbackFontFace(family.family, family.fallback))
 	}
 	return blocks.join('\n\n')
 }
 
-function renderFontFace(family: string, face: FontFace, defaultDisplay: string | undefined): string {
+function renderFontFace(
+	family: string,
+	face: FontFace,
+	defaultDisplay: string | undefined,
+	axes?: FontFamily['axes']
+): string {
 	const src = face.sources
 		.map((s) => `url('${s.url}')${s.format ? ` format('${s.format}')` : ''}`)
 		.join(', ')
+	// Variable fonts: when the face omits an explicit weight and the family declares a
+	// `wght` axis range, emit the range (`font-weight: 100 900`) so a single face spans the
+	// whole weight axis. (TYPOGRAPHY_ENGINE_RFC §5 — variable fonts)
+	const weight = face.weight ?? (axes?.wght ? `${axes.wght[0]} ${axes.wght[1]}` : 400)
 	const lines = [
 		'@font-face {',
 		`\tfont-family: '${family}';`,
 		`\tsrc: ${src};`,
-		`\tfont-weight: ${face.weight ?? 400};`,
+		`\tfont-weight: ${weight};`,
 		`\tfont-style: ${face.style ?? 'normal'};`,
 		`\tfont-display: ${face.display ?? defaultDisplay ?? 'swap'};`
 	]