Add cohort-heatmap-server example

Demo MCP App visualizing cohort retention data with:
- Interactive heatmap with HSL color interpolation
- Multiple metrics: Retention %, Revenue Retention, Active Users
- Monthly/weekly period views
- Cell hover tooltips and row/column highlighting
- Light/dark theme support

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

examples/cohort-heatmap-server/README.md

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+# Example: Cohort Heatmap App
+
+A demo MCP App that displays cohort retention data as an interactive heatmap, showing customer retention over time by signup month.
+
+## Features
+
+- **Cohort Retention Heatmap**: Color-coded grid showing retention percentages across cohorts and time periods
+- **Multiple Metrics**: Switch between Retention %, Revenue Retention, and Active Users
+- **Period Types**: View data by monthly or weekly intervals
+- **Interactive Exploration**: Hover cells for detailed tooltips, click to highlight rows/columns
+- **Color Scale**: Green (high retention) through yellow to red (low retention)
+- **Theme Support**: Adapts to light/dark mode preferences
+
+## Running
+
+1. Install dependencies:
+
+   ```bash
+   npm install
+   ```
+
+2. Build and start the server:
+
+   ```bash
+   npm start
+   ```
+
+   The server will listen on `http://localhost:3001/mcp`.
+
+3. View using the [`basic-host`](https://github.com/modelcontextprotocol/ext-apps/tree/main/examples/basic-host) example or another MCP Apps-compatible host.
+
+## Architecture
+
+### Server (`server.ts`)
+
+Exposes a single `get-cohort-data` tool that returns:
+
+- Cohort rows with signup month, original user count, and retention cells
+- Period headers and labels
+- Configurable parameters: metric type, period type, cohort count, max periods
+
+The tool generates synthetic cohort data using an exponential decay model with configurable retention curves per metric type.
+
+### App (`src/mcp-app.tsx`)
+
+- Uses React for the heatmap visualization
+- Fetches data on mount and when filters change
+- Displays retention percentages in a grid with HSL color interpolation
+- Shows detailed tooltips on hover with user counts and exact retention values
+- Supports row/column highlighting on cell click