feat: display friendly names for memory files (closes #177)

The memory file list now shows human-readable titles instead of raw
filenames wherever possible.

- Rust: adds `MemoryFileInfo { path, heading }` and updates
  `MemoryFilesResponse` to use `Vec<MemoryFileInfo>`; adds
  `extract_first_heading()` helper that scans the first `# Heading`
  from a file's content; both `list_memory_files_native` and
  `list_memory_files_via_wsl` now read each file and populate the
  heading field; raw filename remains available as a fallback
- Frontend: updates `MemoryBrowserPanel.svelte` to use the richer type;
  adds `getDisplayName()` which returns the heading or falls back to the
  raw filename; file buttons use the display name with the raw filename
  as a tooltip; the viewer header also shows the display name with the
  filename shown as a subtitle when a heading is present
- Tests: 8 new Rust unit tests for `extract_first_heading` (happy path,
  edge cases, empty content, whitespace); 12 new TypeScript tests for
  `getFileName` and `getDisplayName` mirrored from the component

src/lib/components/MemoryBrowserPanel.svelte

@@ -3,17 +3,22 @@
   import { invoke } from "@tauri-apps/api/core";
   import Markdown from "./Markdown.svelte";
 
-  let memoryFiles: string[] = $state([]);
+  interface MemoryFileInfo {
+    path: string;
+    heading: string | null;
+  }
+
+  interface MemoryFilesResponse {
+    files: MemoryFileInfo[];
+  }
+
+  let memoryFiles: MemoryFileInfo[] = $state([]);
   let selectedFile: string | null = $state(null);
   let fileContent: string = $state("");
   let isLoading = $state(false);
   let error: string | null = $state(null);
   let isPanelOpen = $state(false);
 
-  interface MemoryFilesResponse {
-    files: string[];
-  }
-
   async function loadMemoryFiles() {
     isLoading = true;
     error = null;
@@ -49,6 +54,10 @@
     return path.split("/").pop() || path;
   }
 
+  function getDisplayName(file: MemoryFileInfo): string {
+    return file.heading ?? getFileName(file.path);
+  }
+
   function togglePanel() {
     isPanelOpen = !isPanelOpen;
     if (isPanelOpen && memoryFiles.length === 0) {
@@ -151,11 +160,12 @@
       {:else}
         <div class="panel-layout">
           <div class="file-list">
-            {#each memoryFiles as file (file)}
+            {#each memoryFiles as file (file.path)}
               <button
                 class="file-item"
-                class:active={selectedFile === file}
-                onclick={() => loadFileContent(file)}
+                class:active={selectedFile === file.path}
+                onclick={() => loadFileContent(file.path)}
+                title={getFileName(file.path)}
               >
                 <svg
                   class="file-icon"
@@ -171,7 +181,7 @@
                     d="M9 12h6m-6 4h6m2 5H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z"
                   />
                 </svg>
-                <span class="file-name">{getFileName(file)}</span>
+                <span class="file-name">{getDisplayName(file)}</span>
               </button>
             {/each}
           </div>
@@ -179,7 +189,12 @@
           <div class="file-viewer">
             {#if selectedFile && fileContent}
               <div class="viewer-header">
-                <h4>{getFileName(selectedFile)}</h4>
+                {#each memoryFiles.filter((f) => f.path === selectedFile) as activeFile (activeFile.path)}
+                  <h4>{getDisplayName(activeFile)}</h4>
+                  {#if activeFile.heading}
+                    <p class="viewer-filename">{getFileName(activeFile.path)}</p>
+                  {/if}
+                {/each}
               </div>
               <div class="viewer-content">
                 <Markdown content={fileContent} />
@@ -438,6 +453,13 @@
     color: var(--text-primary);
   }
 
+  .viewer-filename {
+    margin: 0.25rem 0 0;
+    font-size: 0.75rem;
+    color: var(--text-tertiary);
+    font-family: monospace;
+  }
+
   .viewer-content {
     flex: 1;
     padding: 1.5rem;