test(ui): a component with no stylesheet reads the theme's :root variables

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/library/ui_theme.ludic

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+# ui_theme.ludic — a component with no stylesheet of its own still reads the theme's :root variables,
+# inherited from the screen that uses it, in its style="..." declarations.
+import "ludic.ui"
+import "ui_theme_parts/Bare.ludic"
+import "ui_theme_parts/Shell.ludic"
+program UiTheme {
+  numbers float
+  entry {
+    ui_backend(new UiBackend)
+    let root: UiNode = ui_nodes("Shell", null)
+    ui_place(root, 0.0, 0.0, 400.0, 400.0)
+    let bare = root.children[0].children[0]
+    print(`accent {ui_hex(bare.children[0].fg)} gap {int(bare.gap)}`)
+  }
+}

examples/library/ui_theme_parts/Bare.ludic

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+# Bare.ludic - a component with no stylesheet: it reads the theme's variables from where it is used
+component Bare {
+}

examples/library/ui_theme_parts/Bare.xml

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+<div style="gap: var(--gap)">
+  <p style="color: var(--accent)">hot</p>
+  <p>plain</p>
+</div>

examples/library/ui_theme_parts/Shell.lss

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+@import "theme.lss";
+.shell { width: 300px }

examples/library/ui_theme_parts/Shell.ludic

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+# Shell.ludic - a screen whose sheet imports the theme, around a component with no sheet of its own
+component Shell {
+}

examples/library/ui_theme_parts/Shell.xml

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+<div class="shell">
+  <Bare/>
+</div>

examples/library/ui_theme_parts/theme.lss

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+:root { --accent: #ff0000; --gap: 7px }