feat(mcp): add diagram version history

- Add history.ts module with circular buffer (max 50 entries)
- Add history UI with floating button and modal
- Add HTTP endpoints: /api/history, /api/restore
- Add MCP tools: list_history, restore_version, get_version
- Save history before and after AI changes
- Track source (ai/human) for each entry

packages/mcp-server/src/history.ts

@@ -0,0 +1,169 @@
+/**
+ * Diagram Version History for MCP Server
+ *
+ * Stores diagram versions in-memory per session.
+ * Enables users and AI to restore previous diagram states.
+ */
+
+import { log } from "./logger.js"
+
+export interface HistoryEntry {
+    version: number
+    xml: string
+    svg: string // SVG data for thumbnail preview
+    source: "ai" | "human"
+    tool?: string // Which tool made the change (display_diagram, edit_diagram, browser_sync)
+    timestamp: Date
+    description?: string
+}
+
+interface SessionHistory {
+    entries: HistoryEntry[]
+    nextVersion: number
+}
+
+// In-memory history store keyed by session ID
+const historyStore = new Map<string, SessionHistory>()
+
+// Configuration
+const MAX_HISTORY_ENTRIES = 50
+
+/**
+ * Add a new entry to session history
+ * Returns the assigned version number
+ */
+export function addHistoryEntry(
+    sessionId: string,
+    entry: Omit<HistoryEntry, "version">,
+): number {
+    let history = historyStore.get(sessionId)
+    if (!history) {
+        history = { entries: [], nextVersion: 1 }
+        historyStore.set(sessionId, history)
+    }
+
+    // Deduplicate: skip if XML is identical to last entry
+    const lastEntry = history.entries[history.entries.length - 1]
+    if (lastEntry && lastEntry.xml === entry.xml) {
+        log.debug(`Skipping duplicate history entry for session ${sessionId}`)
+        return lastEntry.version
+    }
+
+    const version = history.nextVersion++
+    const newEntry: HistoryEntry = {
+        ...entry,
+        version,
+    }
+
+    history.entries.push(newEntry)
+
+    // Prune oldest entries if over limit (circular buffer)
+    if (history.entries.length > MAX_HISTORY_ENTRIES) {
+        const removed = history.entries.shift()
+        log.debug(`Pruned oldest history entry v${removed?.version}`)
+    }
+
+    log.info(
+        `Added history v${version} for session ${sessionId} (source: ${entry.source}, entries: ${history.entries.length})`,
+    )
+
+    return version
+}
+
+/**
+ * Get history entries for a session
+ * Returns newest first, limited to specified count
+ */
+export function getHistory(
+    sessionId: string,
+    limit: number = 20,
+): HistoryEntry[] {
+    const history = historyStore.get(sessionId)
+    if (!history) {
+        return []
+    }
+
+    // Return newest first
+    return [...history.entries].reverse().slice(0, limit)
+}
+
+/**
+ * Get a specific version from history
+ */
+export function getVersion(
+    sessionId: string,
+    version: number,
+): HistoryEntry | undefined {
+    const history = historyStore.get(sessionId)
+    if (!history) {
+        return undefined
+    }
+
+    return history.entries.find((e) => e.version === version)
+}
+
+/**
+ * Get the latest version number for a session
+ */
+export function getLatestVersion(sessionId: string): number {
+    const history = historyStore.get(sessionId)
+    if (!history || history.entries.length === 0) {
+        return 0
+    }
+    return history.entries[history.entries.length - 1].version
+}
+
+/**
+ * Clear history for a session (used on session expiry)
+ */
+export function clearHistory(sessionId: string): void {
+    if (historyStore.has(sessionId)) {
+        historyStore.delete(sessionId)
+        log.info(`Cleared history for session ${sessionId}`)
+    }
+}
+
+/**
+ * Update the SVG of the latest entry (or specific version) that has empty SVG
+ * Used when browser generates SVG after loading AI diagram
+ */
+export function updateLatestEntrySvg(
+    sessionId: string,
+    svg: string,
+    targetVersion?: number,
+): boolean {
+    const history = historyStore.get(sessionId)
+    if (!history || history.entries.length === 0) {
+        return false
+    }
+
+    // Find entry to update - either specific version or latest without SVG
+    let entry: HistoryEntry | undefined
+    if (targetVersion !== undefined) {
+        entry = history.entries.find((e) => e.version === targetVersion)
+    } else {
+        // Find most recent entry without SVG
+        for (let i = history.entries.length - 1; i >= 0; i--) {
+            if (!history.entries[i].svg) {
+                entry = history.entries[i]
+                break
+            }
+        }
+    }
+
+    if (entry && !entry.svg) {
+        entry.svg = svg
+        log.debug(`Updated SVG for history v${entry.version}`)
+        return true
+    }
+
+    return false
+}
+
+/**
+ * Get count of history entries for a session
+ */
+export function getHistoryCount(sessionId: string): number {
+    const history = historyStore.get(sessionId)
+    return history?.entries.length || 0
+}