feat(f7.1): mermaid diff dispatcher + type detection (#22)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/mermaidDiff.ts

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+/**
+ * mermaidDiff — F7.1 (#22) dispatcher. Detects a mermaid diagram's type and
+ * routes a baseline→current diff to the matching pure differ, which re-emits the
+ * CURRENT diagram source augmented with mermaid styling directives (INV-29). Any
+ * unsupported type or parser surprise degrades to the v1 whole-block badge
+ * (INV-30) — this function never throws. vscode-free, DOM-free, deterministic.
+ */
+import { diffFlowchart } from "./mermaidFlowchartDiff";
+import { diffSequence } from "./mermaidSequenceDiff";
+
+/** Theme-neutral colors baked into emitted mermaid source (source can't read CSS vars). */
+export const CW_COLORS = {
+  added: "#2ea043",
+  changed: "#d29922",
+  removed: "#808080",
+} as const;
+
+export type DiagramType = "flowchart" | "sequence" | "other";
+
+export interface MermaidDiffAugmented {
+  kind: "augmented";
+  source: string;
+}
+export interface MermaidDiffFallback {
+  kind: "fallback";
+}
+export type MermaidDiffResult = MermaidDiffAugmented | MermaidDiffFallback;
+
+export function detectDiagramType(source: string): DiagramType {
+  for (const line of source.split(/\r?\n/)) {
+    const t = line.trim();
+    if (t === "" || t.startsWith("%%")) continue;
+    if (/^(flowchart|graph)\b/.test(t)) return "flowchart";
+    if (/^sequenceDiagram\b/.test(t)) return "sequence";
+    return "other";
+  }
+  return "other";
+}
+
+export function diffMermaid(beforeSrc: string, currentSrc: string): MermaidDiffResult {
+  const type = detectDiagramType(currentSrc);
+  try {
+    if (type === "flowchart") return { kind: "augmented", source: diffFlowchart(beforeSrc, currentSrc) };
+    if (type === "sequence") return { kind: "augmented", source: diffSequence(beforeSrc, currentSrc) };
+  } catch {
+    return { kind: "fallback" };
+  }
+  return { kind: "fallback" };
+}

src/mermaidFlowchartDiff.ts

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+// stub, replaced in Tasks 2-3
+export function diffFlowchart(_beforeSrc: string, currentSrc: string): string {
+  return currentSrc;
+}

src/mermaidSequenceDiff.ts

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+// stub, replaced in Tasks 4-5
+export function diffSequence(_beforeSrc: string, currentSrc: string): string {
+  return currentSrc;
+}