[Feature] Support Incremental Static Regeneration (ISR) Cache

[HaHiepThanh]

📋 Context & Problem Description

Currently, Gutter supports two primary loading strategies: Dynamic SSR (HTML is rendered dynamically on every HTTP request) and SSG (HTML pages pre-rendered at build time).

For pages with dynamic content that updates periodically but not in real-time (e.g. blog posts, product catalogs), dynamic SSR is CPU-heavy on the host Go server, while static SSG requires rebuilding/redeploying the entire site to reflect a change.

💡 Proposed Solution & Implementation Plan

Implement Incremental Static Regeneration (ISR) caching on the Gutter host server:

1. Configurable Cache Lifetimes:
   Allow defining a cache TTL on SSR routes:

   gutter.SSRConfig{
       Addr: ":8080",
       Root: Root,
       RevalidateSeconds: 60, // Keep static page for 60s
   }

2. Server-Side Page Cache:
   • On request, check if a statically rendered HTML file for the request path exists in the memory/disk cache and check its age.
   • If it is within the TTL, serve the static HTML directly (very low latency, high throughput).
3. Stale-While-Revalidate Regeneration:
   • If the cache is expired, immediately serve the stale static page (ensuring 0ms render latency for the current request).
   • Spawn a background goroutine to execute RenderToHTML on the widget tree, updating the cache with the new HTML for subsequent requests.

🎯 Impact & Benefits

• High Performance: Instant load times (similar to SSG) for the vast majority of requests.
• Low Server Overhead: Drastically reduces server rendering CPU usage under heavy traffic.

🏷️ Suggested Labels

feature, ssr, performance