docs: HTMX example app demonstrating tavern features

[catgoose]

Summary

Create a working HTMX example application that demonstrates tavern's key features in a realistic setting. This serves as both documentation and a proof-of-concept for new users evaluating the library.

Proposed App

A live dashboard application (e.g., project management board, monitoring dashboard, or team activity feed) that showcases:

Core features:

• Topic-based pub/sub with SSE streaming
• Scoped subscriptions (per-user views)
• OOB fragment swaps (Replace, Append, Delete)
• Snapshot + delta streams (initial state + live updates)

Advanced features:

• Topic groups (single SSE connection per page)
• Reactive hooks (After hooks cascading updates across UI regions)
• Mutation hooks (form submit triggers SSE updates to other viewers)
• Subscriber filtering (role-based content)
• TTL messages (toast notifications that auto-expire)
• Presence tracking (who's viewing)
• Adaptive backpressure (graceful degradation demo)
• Batch publish (bulk actions updating multiple regions atomically)

Stack:

• Go + net/http (or Echo)
• HTMX with SSE extension
• Minimal CSS (Pico, Simple.css, or similar)
• No build step, no JS framework

Location

_examples/ directory in the repo. Self-contained with its own main.go and go.mod (or use the parent module).

Goals

• New users can go run . and see tavern working in under 30 seconds
• Each feature is demonstrated with inline comments explaining the pattern
• Serves as a living integration test