[repo-assist] perf: index related section lookups (#1416)

🤖 *This is an automated PR from Repo Assist, an AI assistant for this
repository.*

## Summary

`FindRelatedSections` previously searched every site mapping for each
semantic result. This change builds a per-request chapter/heading index
once and uses dictionary lookups, reducing the mapping phase from
O(results × mappings) to O(mappings + results) while preserving
case-insensitive matching and first-match behavior.

## Root cause and rationale

Related-section results performed a linear scan of the complete
site-mapping collection for every result. As the book mapping grows,
that repeated work scales unnecessarily. The local index keeps the
existing result and fallback behavior but avoids repeated enumeration.

## Test Status

- ✅ `dotnet build EssentialCSharp.Web/EssentialCSharp.Web.csproj
--configuration Release --no-restore`
- ⚠️ `dotnet test
EssentialCSharp.Web.Tests/EssentialCSharp.Web.Tests.csproj
--configuration Release --no-restore`: 149 passed, 2 pre-existing
external integration failures (`CaptchaService_Verify_Success` and
`KnownBreachedPassword_IsDetected`) because their third-party services
were unavailable in this environment.
- ✅ `git diff --check`

No new dependencies were added.

> [!WARNING]
> <details>
> <summary>Firewall blocked 2 domains</summary>
>
> The following domains were blocked by the firewall during workflow
execution:
>
> - `api.hcaptcha.com`
> - `api.pwnedpasswords.com`
>
> To allow these domains, add them to the `network.allowed` list in your
workflow frontmatter:
>
> ```yaml
> network:
>   allowed:
>     - defaults
>     - "api.hcaptcha.com"
>     - "api.pwnedpasswords.com"
> ```
>
> See [Network
Configuration](https://github.github.com/gh-aw/reference/network/) for
more information.
>
> </details>

> Generated by 🌈 Repo Assist, see [workflow
run](https://github.com/IntelliTect/EssentialCSharp.Web/actions/runs/36033164072).
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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

EssentialCSharp.Web/Tools/BookSearchTool.cs

@@ -322,16 +322,29 @@ public async Task<string> FindRelatedSections(
 
         List<RelatedSectionMatchTextResult> relatedSections = [];
         var seen = new HashSet<string>(StringComparer.OrdinalIgnoreCase) { mapping.RawHeading };
+        Dictionary<int, Dictionary<string, SiteMapping>> mappingsByChapter = [];
+        foreach (SiteMapping candidate in _SiteMappingService.SiteMappings)
+        {
+            if (!mappingsByChapter.TryGetValue(candidate.ChapterNumber, out Dictionary<string, SiteMapping>? chapterMappings))
+            {
+                chapterMappings = new Dictionary<string, SiteMapping>(StringComparer.OrdinalIgnoreCase);
+                mappingsByChapter[candidate.ChapterNumber] = chapterMappings;
+            }
+
+            chapterMappings.TryAdd(candidate.RawHeading, candidate);
+        }
+
         foreach (var r in results)
         {
             string heading = r.Record.Heading ?? "";
             if (!seen.Add(heading)) continue;
             if (relatedSections.Count >= maxResults) break;
 
             // Find the SiteMapping for this heading to get the link
-            SiteMapping? relatedMapping = _SiteMappingService.SiteMappings
-                .FirstOrDefault(m => m.RawHeading.Equals(heading, StringComparison.OrdinalIgnoreCase)
-                                  && m.ChapterNumber == (r.Record.ChapterNumber ?? 0));
+            SiteMapping? relatedMapping = mappingsByChapter.TryGetValue(r.Record.ChapterNumber ?? 0, out Dictionary<string, SiteMapping>? chapterMappings)
+                && chapterMappings.TryGetValue(heading, out SiteMapping? candidate)
+                ? candidate
+                : null;
 
             string link = relatedMapping is not null
                 ? $"`/{relatedMapping.Keys.FirstOrDefault() ?? relatedMapping.PrimaryKey}#{relatedMapping.AnchorId}`"