feat(select, combobox): plane-constant popup elevation (B2)

Per the elevation doctrine (variant = emphasis, elevation = plane), the
select / combobox popups now take their surface · border · shadow from the
`overlay` plane via `data-depth="overlay"` — constant across variants. The
variant (surface / outline / ghost) tints only the trigger / control, no
longer the popup elevation.

- Drop the content's own border / bg / shadow + the per-variant elevation
  overrides (outline/ghost were `raised` / lighter / borderless).
- Point the arrows at `--depth-overlay-surface` / `--depth-overlay-border`.
- Prune select's 6 now-orphaned `content-*` elevation recipe tokens.

Verified in-browser (measured): combobox surface AND ghost both paint
bg oklch(0.285) + 1px border + overlay shadow from the plane (ghost was
transparent/borderless before — now flattened to the plane). select is the
identical change; its open popup hangs the preview renderer (no console
errors — preview infra), so it rides the same plane mechanism + check 0 +
eidos-lint 0 invalid + contract test (no new orphans).

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

src/uix/eidos/components/combobox/combobox-content.svelte

@@ -39,6 +39,7 @@
 	customAnchor={anchorEl}
 	style={composedStyle}
 	data-list-surface=""
+	data-depth="overlay"
 	data-size={resolvedSize}
 	data-variant={resolvedVariant}
 	data-color={resolvedColor}

src/uix/eidos/components/combobox/combobox.css

@@ -275,10 +275,10 @@
 	max-block-size: min(22rem, var(--floating-available-height, 100dvh));
 	padding: var(--_combobox-content-py) var(--_combobox-content-px);
 	overflow: hidden;
-	border: var(--border-width) solid var(--color-border-default);
+	/* Elevation — surface · border · shadow — from the `overlay` plane via
+	   `data-depth` (A1). Constant across variants (the variant tints only the
+	   control). Radius / color stay the panel's own. */
 	border-radius: var(--radius-lg);
-	background: var(--color-surface-overlay);
-	box-shadow: var(--depth-overlay-shadow), var(--depth-overlay-halo);
 	color: var(--color-content-primary);
 	font-family: var(--font-ui);
 	font-size: var(--_combobox-font-size);
@@ -311,14 +311,8 @@
 	--_combobox-font-size: var(--font-size-xl);
 }
 
-[data-combobox-content][data-variant='outline'] {
-	box-shadow: var(--depth-raised-shadow), var(--depth-raised-halo);
-}
-
-[data-combobox-content][data-variant='ghost'] {
-	border-color: transparent;
-	box-shadow: var(--depth-raised-shadow), var(--depth-raised-halo);
-}
+/* Variant no longer changes the popup elevation — it is plane-constant (A1).
+   `surface` / `outline` / `ghost` tint only the control. */
 
 [data-combobox-content][data-state='open'] {
 	animation: combobox-content-in var(--duration-normal) var(--ease-out) both;
@@ -464,12 +458,12 @@
 }
 
 [data-combobox-arrow] polygon {
-	fill: var(--color-surface-overlay);
+	fill: var(--depth-overlay-surface);
 	stroke: none;
 }
 
 [data-combobox-arrow] path {
-	stroke: var(--color-border-default);
+	stroke: var(--depth-overlay-border);
 	stroke-width: var(--border-width);
 	stroke-linecap: round;
 	stroke-linejoin: round;

src/uix/eidos/components/select/select-content.svelte

@@ -34,6 +34,7 @@
 	{...rest}
 	style={composedStyle}
 	data-list-surface=""
+	data-depth="overlay"
 	data-size={resolvedSize}
 	data-variant={resolvedVariant}
 	data-color={resolvedColor}

src/uix/eidos/components/select/select.css

@@ -177,10 +177,10 @@
 	max-block-size: min(var(--select-content-max-height), var(--floating-available-height, 100dvh));
 	padding: var(--_select-content-py) var(--_select-content-px);
 	overflow: hidden;
-	border: var(--select-content-border-width) solid var(--select-content-border);
+	/* Elevation — surface · border · shadow — from the `overlay` plane via
+	   `data-depth` (A1). Constant across variants (the variant tints only the
+	   trigger). Radius / color stay the panel's own. */
 	border-radius: var(--select-content-radius);
-	background: var(--select-content-bg);
-	box-shadow: var(--select-content-shadow);
 	color: var(--select-content-color);
 	font-family: var(--select-font-family);
 	font-size: var(--_select-font-size);
@@ -213,14 +213,8 @@
 	--_select-font-size: var(--select-font-size-xl);
 }
 
-[data-select-content][data-variant='outline'] {
-	box-shadow: var(--select-content-outline-shadow);
-}
-
-[data-select-content][data-variant='ghost'] {
-	border-color: transparent;
-	box-shadow: var(--select-content-ghost-shadow);
-}
+/* Variant no longer changes the popup elevation — it is plane-constant (A1).
+   `surface` / `outline` / `ghost` tint only the trigger. */
 
 [data-select-content][data-state='open'] {
 	animation: select-content-in var(--select-open-duration) var(--ease-out) both;
@@ -365,13 +359,13 @@
 }
 
 [data-select-arrow] polygon {
-	fill: var(--select-content-bg);
+	fill: var(--depth-overlay-surface);
 	stroke: none;
 }
 
 [data-select-arrow] path {
-	stroke: var(--select-content-border);
-	stroke-width: var(--select-content-border-width);
+	stroke: var(--depth-overlay-border);
+	stroke-width: var(--border-width);
 	stroke-linecap: round;
 	stroke-linejoin: round;
 	vector-effect: non-scaling-stroke;

src/uix/eidos/generated/base.css

@@ -2189,14 +2189,8 @@
 	--select-content-min-width: 12rem;
 	--select-content-max-width: 24rem;
 	--select-content-max-height: min(20rem, calc(100dvh - var(--space-6)));
-	--select-content-border-width: var(--border-width);
-	--select-content-border: var(--color-border-default);
 	--select-content-radius: var(--radius-lg);
-	--select-content-bg: var(--color-surface-overlay);
 	--select-content-color: var(--color-content-primary);
-	--select-content-shadow: var(--depth-overlay-shadow), var(--depth-overlay-halo);
-	--select-content-outline-shadow: var(--depth-raised-shadow), var(--depth-raised-halo);
-	--select-content-ghost-shadow: var(--depth-raised-shadow), var(--depth-raised-halo);
 	--select-open-duration: var(--duration-normal);
 	--select-close-duration: var(--duration-fast);
 	--select-viewport-gap: var(--space-0-5);

src/uix/eidos/lib/recipes/base.ts

@@ -2254,14 +2254,8 @@ export const THEME_BASE_RECIPE_TOKENS = {
 		'content-min-width': '12rem',
 		'content-max-width': '24rem',
 		'content-max-height': 'min(20rem, calc(100dvh - var(--space-6)))',
-		'content-border-width': 'var(--border-width)',
-		'content-border': 'var(--color-border-default)',
 		'content-radius': 'var(--radius-lg)',
-		'content-bg': 'var(--color-surface-overlay)',
 		'content-color': 'var(--color-content-primary)',
-		'content-shadow': 'var(--depth-overlay-shadow), var(--depth-overlay-halo)',
-		'content-outline-shadow': 'var(--depth-raised-shadow), var(--depth-raised-halo)',
-		'content-ghost-shadow': 'var(--depth-raised-shadow), var(--depth-raised-halo)',
 		'open-duration': 'var(--duration-normal)',
 		'close-duration': 'var(--duration-fast)',
 		'viewport-gap': 'var(--space-0-5)',