Add landing screen as the app's entry point

Introduce a simple, show-don't-tell entry screen that lowers the "what
counts as a protocol" barrier and frames the activity as a short journey:

- Lead line "Examine the protocols around you" over a rotating, cross-fading
  list of deliberately diverse examples (traffic, code, kitchens, ritual,
  governance, play)
- Prominent "Begin" CTA above a three-step path: Describe / Understand / Share
- "About the Bicorder" link opens the existing help modal

Shown on first arrival; returning users with a reading in progress skip
straight to the diagnostic (computed synchronously, no flash). Remove the
now-redundant inline description from the focused flow.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

bicorder-app/src/App.svelte

@@ -9,6 +9,7 @@
   import FormRecommendation from './components/FormRecommendation.svelte';
   import AnalysisTransitionBanner from './components/AnalysisTransitionBanner.svelte';
   import HamburgerMenu from './components/HamburgerMenu.svelte';
+  import Landing from './components/Landing.svelte';
   import { BicorderClassifier } from './bicorder-classifier';
 
   // Load bicorder data and model from build-time constants
@@ -30,6 +31,32 @@
   let refreshKey = 0; // Used to force component refresh in focused mode
   let isHelpOpen = false;
 
+  // Show the landing screen on first arrival. Returning users who already have
+  // a reading in progress skip straight to the diagnostic. Computed
+  // synchronously (not in onMount) so the landing never flashes for them.
+  function hasReadingInProgress(): boolean {
+    if (typeof localStorage === 'undefined') return false;
+    try {
+      const saved = localStorage.getItem('bicorder-state');
+      if (!saved) return false;
+      const s = JSON.parse(saved);
+      const hasProtocol = !!s?.metadata?.protocol;
+      const hasValue = s?.diagnostic?.some(
+        (set: any) => set?.gradients?.some((g: any) => g?.value !== null && g?.value !== undefined)
+      );
+      return hasProtocol || hasValue;
+    } catch {
+      return false;
+    }
+  }
+
+  let started = hasReadingInProgress();
+
+  function startReading() {
+    started = true;
+    currentScreen = 0;
+  }
+
   // Screen types
   type Screen =
     | { type: 'metadata' }
@@ -432,6 +459,9 @@
 
 <HelpModal bind:isOpen={isHelpOpen} />
 
+{#if !started}
+  <Landing on:begin={startReading} on:about={openHelp} />
+{:else}
 <main>
   <div class="header">
     <div class="header-left">
@@ -519,12 +549,6 @@
 
   {:else}
     <!-- FOCUSED MODE: Show one screen at a time -->
-    {#if currentScreen === 0}
-      <div class="description">
-        <p>A diagnostic tool for the study of protocols</p>
-      </div>
-    {/if}
-
     <div class="focused-container">
       {#if currentScreenData.type === 'metadata'}
         <div class="focused-screen">
@@ -679,6 +703,7 @@
     </div>
   {/if}
 </main>
+{/if}
 
 <style>
   main {