Feat/reactive islands

[hexplus]

Description

Adds reactive HTML islands to SibuJS — a third rendering mode that attaches fine-grained reactivity to server-rendered HTML with no build step, no JSX, no virtual DOM, and no dependencies. The whole islands runtime is ~5.2 KB gzipped. Additive and backward-compatible — nothing existing changes.

• enhance(target, setup) — attach reactivity to existing DOM without re-rendering it (alongside mount and hydrate, which replace markup). The EnhanceContext drives server markup in place: text, attr (a11y-correct literal booleans), classed, show (toggles the standard hidden property, so it reveals server-hidden elements), model (two-way; checkbox / number / <select multiple>), on, ref/refs, cleanup. Targets resolve via @name → data-ref="name" or a raw selector. Returns a dispose tied to the element. Plus enhanceAll(selector, setup).
• Island runtime — registerIsland(name, setup) + mountIslands(root?). Declare islands with data-sibu-island="name" and choose when each activates via data-sibu-load: load · idle · visible · interaction · media.
• Lazy island code — lazyIsland(() => import("./island.js")) fetches a module only when its island activates, so a page ships ~0 JS for islands that never trigger.
• Robustness — no-op writes skipped (no hydration flash / re-paint of static content); re-enhancing an element is refused; a failing island's setup is isolated; failed lazy imports are reported, not thrown.
• Exported from the package root and the window.Sibu CDN global → runs from a single <script> tag.
• Docs/examples: docs/islands.md, runnable examples/islands.html + examples/islands-strategies.html, and bench/islands-size.mjs (the 5.2 KB measurement).

Tests: 30 new jsdom unit tests (full suite 3997 passing) plus a real-browser Playwright suite — 15/15 across Chromium, Firefox, and WebKit validating every strategy (real IntersectionObserver / requestIdleCallback / matchMedia / interaction timing) and lazy import() over HTTP. CHANGELOG [3.4.0] added; version bumped to 3.4.0.

Related Issue

Closes #

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• I have read CONTRIBUTING.md
• My code builds without errors
• I have tested my changes
• I have updated documentation if needed