Experimental Astro incremental static builds

<section class="stage"><div class="heading">INCREMENTAL BUILD <span class="hint">code hash + data key</span></div><div class="build-grid"><div class="build-page"><b>PAGE A</b><span>same</span></div><div class="build-page"><b>PAGE B</b><span>changed</span></div><div class="build-page"><b>PAGE C</b><span>same</span></div></div><div class="build-status" id="build-status">Compare previous build keys</div><div class="caption">Only changed paths need another render</div></section>

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