Remove all hotlinked Unsplash images — download locally, replace 14 404s

- Downloaded all 31 Unsplash images to photos/ directory
- Found 14 URLs returning 404; replaced with working equivalents:
  about: book-room, hard-drives, forest-child
  human-body: digestive-system, human-eye
  inventions: gears (x2), lab-glassware
  maps/phenomena: old-map (x2), ornate-globe, earth-orbit
  natural-world: migrating-birds
  phenomena: lightning-storm, ferrofluid
  space: voyager-record
- Updated all 8 HTML files to reference local photos/ instead of Unsplash CDN
- Fixed broken ammonite clipart reference (fossil -> existing image)
- Updated CSS clipart hover effects and responsive rules

natural-world.html

@@ -51,7 +51,7 @@
       <img src="img/acorns-illustration.png" alt="" class="clipart-left">
 
       <div class="figure figure-right">
-        <img src="https://images.unsplash.com/photo-1448375240586-882707db888b?w=500&h=375&fit=crop" alt="Sunlight filtering through an old-growth forest canopy" width="500" height="375">
+        <img src="photos/1448375240586-882707db888b_500x375.jpg" alt="Sunlight filtering through an old-growth forest canopy" width="500" height="375">
         <p class="figure-caption">Old-growth canopy in the Pacific Northwest. Some trees in this photograph germinated before the Magna Carta was signed.</p>
       </div>
 
@@ -74,7 +74,7 @@
       <img src="img/seashell-worm-cartoon.png" alt="" class="clipart-right">
 
       <div class="figure figure-left">
-        <img src="https://images.unsplash.com/photo-1559825481-12a05cc00344?w=500&h=375&fit=crop" alt="A deep ocean scene with bioluminescent creatures" width="500" height="375">
+        <img src="photos/1559825481-12a05cc00344_500x375.jpg" alt="A deep ocean scene with bioluminescent creatures" width="500" height="375">
         <p class="figure-caption">The mesopelagic zone, 200–1,000 meters below the surface, where the last traces of sunlight fade and bioluminescence becomes the dominant light source.</p>
       </div>
 
@@ -93,7 +93,7 @@
 
     <hr class="dk-rule">
 
-    <img src="img/ammonite-shell-fossil-image.png" alt="" class="clipart-wide">
+    <img src="img/shark-tooth-anatomy-drawing.png" alt="" class="clipart-wide">
 
     <img src="img/botanical-flower-fruit-chart.png" alt="" class="clipart-wide">
 
@@ -105,7 +105,7 @@
       <p>No organism exists in isolation. Symbiosis — from the Greek <em>syn</em> (together) and <em>biōsis</em> (living) — describes the close, long-term interactions between different biological species. These relationships fall along a spectrum from mutually beneficial to parasitic, and the natural world is riddled with arrangements so intricate that it becomes impossible to say where one organism ends and another begins.</p>
 
       <div class="figure">
-        <img src="https://images.unsplash.com/photo-1546026423-cc4642628d2b?w=600&h=400&fit=crop" alt="A vibrant coral reef teeming with symbiotic life" width="600" height="400">
+        <img src="photos/1546026423-cc4642628d2b_600x400.jpg" alt="A vibrant coral reef teeming with symbiotic life" width="600" height="400">
         <p class="figure-caption">Coral polyps house photosynthetic zooxanthellae within their tissues, gaining up to 90% of their energy from these microscopic algae. When the algae are expelled — a phenomenon called bleaching — the coral starves.</p>
       </div>
 
@@ -154,7 +154,7 @@
       <p>Migration is not merely movement — it is a fixed, inherited pattern of seasonal travel that rivals any human voyage in endurance and far exceeds it in precision. The Arctic tern makes a round-trip of roughly 71,000 kilometers each year, flying from Arctic to Antarctic and back, experiencing two summers and more daylight than any other creature. Over its lifetime, a single tern may fly the equivalent of three trips to the moon and back.</p>
 
       <div class="figure figure-right">
-        <img src="https://images.unsplash.com/photo-1500332929257-680a68d0e0f1?w=500&h=375&fit=crop" alt="A flock of migrating birds over wetlands" width="500" height="375">
+        <img src="photos/photo-1623328407791-27e8247332e1_500x375.jpg" alt="A flock of migrating birds over wetlands" width="500" height="375">
         <p class="figure-caption">Migratory birds follow routes refined over millions of years, guided by magnetic fields, star patterns, polarized light, and olfactory maps.</p>
       </div>
 
@@ -205,7 +205,7 @@
 
       <p>The extraordinary is not always far away or deep below. It is here, in the photosynthetic alchemy turning sunlight into sugar, in the mycorrhizal internet underfoot, in the barometric instincts of a storm-approaching flock. The natural world is not merely something to visit — it is something we are inside of, permanently, whether we remember it or not.</p>
 
-      <img src="img/rainforest-root-system-illustration.png" alt="" class="clipart-divider">
+      <img src="img/jungle-exploration-illustration.png" alt="" class="clipart-divider">
 
       <p><em>Continue to <a href="inventions.html">Chapter Two: Inventions</a> &rarr;</em></p>
     </div>
@@ -223,4 +223,4 @@
 
   <script src="js/main.js"></script>
 </body>
-</html>
+</html>