Samplers: nav on page background, all sections start collapsed

Review feedback: remove the white background from the nav, and start
all the sections collapsed.

The header now sits on the page background in every direction that
painted it (Primer, Control Room, Current; Swiss never had one) -
Current also drops the negative-margin full-bleed trick that existed
only to carry that fill. Zone 3 panels (notes, discoveries, tasks,
activity) all start collapsed in all four samplers, so the landing
view is composer, needs, and queue health with quiet headers below.

mocks/README.md

@@ -39,9 +39,8 @@ so the samplers stay standalone while rendering crisp at 8rem.
   to wake, an open question; then running / stuck / queued counts and
   saddle states.
 - Zone 3 (ambient, reduced density): notes, latest discoveries,
-  active tasks table and activity timeline — collapsed `<details>`
-  where sensible (tasks and activity start collapsed; notes and
-  discoveries open).
+  active tasks table and activity timeline — ALL sections start
+  collapsed (`<details>` without `open`).
 
 Sample data uses the real app's vocabulary: statuses
 draft / queued / assigned / working / blocked / review_requested /

mocks/dashboard-control.html

@@ -58,7 +58,6 @@ h1, h2, .hint, .tag, .metric dt, .when, dt, th, .sampler-note, .who { font-famil
 .top {
     display: flex; align-items: center; gap: 1.25rem;
     border-bottom: 1px solid var(--rule-hi);
-    background: var(--surface);
     padding: 0.7rem 1.25rem;
 }
 .brand {
@@ -426,7 +425,7 @@ main { max-width: 70rem; margin: 0 auto; padding: 1.25rem 1.25rem 3rem; }
 
     <!-- ZONE 3: ambient. Reduced density, collapsed where sensible. -->
     <section class="zone-ambient" aria-label="Ambient">
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Notes</h2><span class="summary-meta">3</span></summary>
             <ul class="notes">
                 <li class="note open"><span class="note-state"></span>
@@ -451,7 +450,7 @@ main { max-width: 70rem; margin: 0 auto; padding: 1.25rem 1.25rem 3rem; }
             </ul>
         </details>
 
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Latest discoveries</h2><span class="summary-meta">2</span></summary>
             <ul class="discoveries">
                 <li>

mocks/dashboard-current.html

@@ -67,9 +67,6 @@ a:hover { text-decoration: underline; }
     gap: var(--grid-2);
     border-bottom: 1px solid var(--line);
     padding: var(--grid-2) 0;
-    background: var(--surface);
-    margin: 0 calc(-1 * var(--grid-2));
-    padding-inline: var(--grid-2);
 }
 .brand {
     margin: 0; font-size: 1.25rem; font-weight: 650;
@@ -400,7 +397,7 @@ main { padding: var(--grid-2) 0 var(--grid-4); }
 
     <!-- ZONE 3: ambient. Reduced density, collapsed where sensible. -->
     <section class="zone-ambient" aria-label="Ambient">
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Notes</h2><span class="summary-meta">3</span></summary>
             <ul class="notes">
                 <li class="note open"><span class="note-state"></span>
@@ -425,7 +422,7 @@ main { padding: var(--grid-2) 0 var(--grid-4); }
             </ul>
         </details>
 
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Latest discoveries</h2><span class="summary-meta">2</span></summary>
             <ul class="discoveries">
                 <li>

mocks/dashboard-primer.html

@@ -57,7 +57,6 @@ a:hover { text-decoration: underline; }
     display: flex; align-items: center; gap: 1rem;
     max-width: 66rem; margin: 0 auto; padding: 0.9rem 1rem 0.7rem;
     border-bottom: 1px solid var(--line);
-    background: var(--surface);
 }
 .brand {
     margin: 0; font-size: 1rem; font-weight: 650; letter-spacing: -0.01em;
@@ -437,7 +436,7 @@ main { max-width: 66rem; margin: 0 auto 3rem; padding: 0 1rem; }
 
     <!-- ZONE 3: ambient. Reduced density, collapsed where sensible. -->
     <section class="zone-ambient" aria-label="Ambient">
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Notes</h2><span class="summary-meta">3</span></summary>
             <ul class="notes">
                 <li class="note open"><span class="note-state"></span>
@@ -462,7 +461,7 @@ main { max-width: 66rem; margin: 0 auto 3rem; padding: 0 1rem; }
             </ul>
         </details>
 
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Latest discoveries</h2><span class="summary-meta">2</span></summary>
             <ul class="discoveries">
                 <li>

mocks/dashboard-swiss.html

@@ -419,7 +419,7 @@ main {
 
     <!-- ZONE 3: ambient. Reduced density, collapsed where sensible. -->
     <section class="zone-ambient" aria-label="Ambient">
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Notes</h2><span class="summary-meta">3</span></summary>
             <ul class="notes">
                 <li class="note open"><span class="note-state"></span>
@@ -444,7 +444,7 @@ main {
             </ul>
         </details>
 
-        <details class="panel" open>
+        <details class="panel">
             <summary><h2>Latest discoveries</h2><span class="summary-meta">2</span></summary>
             <ul class="discoveries">
                 <li>