feat: the debug dump reports the colour actually painted

The colour section only read the marks, so a colour no mark carries —
a decoration's, a stylesheet rule's, the page's own — was invisible in
the dump. `painted` adds the computed colour per rendered run with the
element that declares it, the page colour variables read at the editor's
own element, and the document stylesheet's colour rules.

src/lib/components/CLAUDE.md

@@ -141,7 +141,7 @@ ceiling noted in the file.
 
 ## Debug tooling (dev only)
 
-In dev builds a **Debug** button (`App.svelte`) downloads a JSON snapshot combining `getPageBreakDebug(view)` (leaves, placements, rendered spacers, table-break bands, live overlay geometry), `getTableCellDebug(view)` (per cell: the `verticalAlign` attr beside the computed value, cell/row height, the gap above and below the content, and every block's margins/`--space-before` — cell alignment reads as broken whenever the content fills the box, so the numbers that decide that travel with it), `getFrameDebug(view)` (`caption.ts` — per picture: its attrs, its rendered span in the text column and its caption's; every other section measures the flow downwards, and a misplaced caption is a sideways question), the style sheet + spacing model, and `getColorDebug(editor)` (selection marks, text runs, document colors, DOM spans).
+In dev builds a **Debug** button (`App.svelte`) downloads a JSON snapshot combining `getPageBreakDebug(view)` (leaves, placements, rendered spacers, table-break bands, live overlay geometry), `getTableCellDebug(view)` (per cell: the `verticalAlign` attr beside the computed value, cell/row height, the gap above and below the content, and every block's margins/`--space-before` — cell alignment reads as broken whenever the content fills the box, so the numbers that decide that travel with it), `getFrameDebug(view)` (`caption.ts` — per picture: its attrs, its rendered span in the text column and its caption's; every other section measures the flow downwards, and a misplaced caption is a sideways question), the style sheet + spacing model, and `getColorDebug(editor)` (selection marks, text runs, document colors, DOM spans, and `painted`: the colour the browser actually computes per rendered run, the element that declares it, the page's colour variables and the document stylesheet's colour rules — a mark says what was asked for, and a decoration paints a colour no mark carries).
 
 ## Per-section header/footer
 

src/lib/utils/colorDebug.ts

@@ -30,6 +30,23 @@ export type ColorDebugSnapshot = {
     total: number;
     sample: Array<{ dataColor: string | null; inlineStyle: string | null; text: string }>;
   };
+  /** What the browser paints — the marks above only say what it was asked to. */
+  painted: {
+    /** The custom properties a run of no colour of its own resolves through. */
+    vars: Record<string, string>;
+    /** One entry per colour actually rendered, with where that colour comes from. */
+    colors: Record<string, { chars: number; sample: string; block: string; source: PaintSource | null }>;
+    /** The colour-carrying rules of the document stylesheet (styles/styleSheet.ts). */
+    styleRules: string[];
+  };
+};
+
+/** The nearest element declaring a colour, i.e. the channel a painted colour arrives on. */
+type PaintSource = {
+  tag: string;
+  className: string;
+  dataColor: string | null;
+  inlineColor: string | null;
 };
 
 const TEXT_PREVIEW_LIMIT = 80;
@@ -55,6 +72,67 @@ function getTextStyleAttr(marks: readonly Mark[], attr: string): string | null {
   return typeof v === 'string' && v ? v : null;
 }
 
+const PAINT_VARS = ['--color-page-text', '--color-page-bg', '--color-text', '--font-color'];
+const INLINE_COLOR = /(?:^|;)\s*color\s*:\s*([^;]+)/;
+const STYLE_RULE_LIMIT = 40;
+
+// Walking up to the editor root: the first element declaring a colour is the channel the
+// run is painted through. None of them = the page's own colour (editor.css).
+function paintSource(from: HTMLElement, root: HTMLElement): PaintSource | null {
+  for (let e: HTMLElement | null = from; e; e = e.parentElement) {
+    const inline = INLINE_COLOR.exec(e.getAttribute('style') ?? '');
+    if (inline || e.hasAttribute('data-color')) {
+      return {
+        tag: e.tagName.toLowerCase(),
+        className: e.getAttribute('class') ?? '',
+        dataColor: e.getAttribute('data-color'),
+        inlineColor: inline ? inline[1].trim() : null,
+      };
+    }
+    if (e === root) break;
+  }
+  return null;
+}
+
+// The block a run sits in, named as the CSS would address it: its tag and the named
+// paragraph style whose rule reaches it.
+function blockOf(from: HTMLElement, root: HTMLElement): string {
+  const block = from.closest('p, h1, h2, h3, h4, h5, h6, h7, h8, h9, h10, li, td, th') ?? from;
+  const styled = from.closest('[data-style]');
+  const name = styled && root.contains(styled) ? styled.getAttribute('data-style') : null;
+  return name ? `${block.tagName.toLowerCase()}[${name}]` : block.tagName.toLowerCase();
+}
+
+function paintedColors(root: HTMLElement): ColorDebugSnapshot['painted']['colors'] {
+  const out: ColorDebugSnapshot['painted']['colors'] = {};
+  if (typeof document === 'undefined' || typeof getComputedStyle !== 'function') return out;
+  const walker = document.createTreeWalker(root, NodeFilter.SHOW_TEXT);
+  for (let n = walker.nextNode(); n; n = walker.nextNode()) {
+    const text = n.textContent ?? '';
+    const el = n.parentElement;
+    if (!text.trim() || !el) continue;
+    const color = getComputedStyle(el).color;
+    if (!out[color]) {
+      out[color] = { chars: 0, sample: truncate(text.trim(), 40), block: blockOf(el, root), source: paintSource(el, root) };
+    }
+    out[color].chars += text.length;
+  }
+  return out;
+}
+
+function documentStyleColorRules(): string[] {
+  const sheet = (document.getElementById('document-styles') as HTMLStyleElement | null)?.sheet;
+  if (!sheet) return [];
+  try {
+    return Array.from(sheet.cssRules)
+      .map((r) => r.cssText)
+      .filter((t) => INLINE_COLOR.test(t) || t.includes('{color:') || t.includes(' color:'))
+      .slice(0, STYLE_RULE_LIMIT);
+  } catch {
+    return [];
+  }
+}
+
 export function getColorDebug(editor: Editor): ColorDebugSnapshot {
   const { state } = editor;
   const { doc, selection, storedMarks } = state;
@@ -115,6 +193,15 @@ export function getColorDebug(editor: Editor): ColorDebugSnapshot {
       ? document.documentElement.getAttribute('data-theme')
       : null;
 
+  // Read on the editor's own element: a page decoration or a theme override lands
+  // between :root and the text, and only this end of the cascade shows it.
+  const paintVars: Record<string, string> = {};
+  if (root && typeof getComputedStyle === 'function') {
+    const cs = getComputedStyle(root as HTMLElement);
+    for (const v of PAINT_VARS) paintVars[v] = cs.getPropertyValue(v).trim();
+    paintVars.color = cs.color;
+  }
+
   return {
     timestamp: new Date().toISOString(),
     theme: { dataTheme },
@@ -133,5 +220,10 @@ export function getColorDebug(editor: Editor): ColorDebugSnapshot {
       total: domSpansTotal,
       sample: domSpansSample,
     },
+    painted: {
+      vars: paintVars,
+      colors: root ? paintedColors(root as HTMLElement) : {},
+      styleRules: typeof document !== 'undefined' ? documentStyleColorRules() : [],
+    },
   };
 }