theory pages added

theory/hex.en.html

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+<h3>Base: 16</h3>
+<p>The hexadecimal (base-16) numeral system uses sixteen symbols: the digits 0–9 and the letters A–F (representing values 10–15).</p>
+<h3>How it works</h3>
+<div class="theory-example">1A3 = 1×256 + 10×16 + 3×1 = 419</div>
+<h3>Digits</h3>
+<div class="theory-example">0  1  2  3  4  5  6  7  8  9  A  B  C  D  E  F</div>
+<h3>Uses</h3>
+<p>Hexadecimal is ubiquitous in computing. Memory addresses, color codes in CSS (#E94560), MAC addresses, and file hashes all use hex because each hex digit maps to exactly four binary digits, making it a compact and human-readable representation of binary data.</p>