feat: footnote anchor navigation (#1245)

## Summary: Enable footnote anchor navigation in EPUB reader

This PR extracts the core anchor-to-page mapping mechanism from PR #1143
(TOC fragment navigation) to provide immediate footnote navigation
support. By merging this focused subset first, users get a complete
footnote experience now while simplifying the eventual review and merge
of the full #1143 PR.

  ---

## What this extracts from PR #1143

PR #1143 implements comprehensive TOC fragment navigation for EPUBs with
multi-chapter spine files. This PR takes only the anchor resolution
infrastructure:

- Anchor-to-page mapping in section cache: During page layout,
ChapterHtmlSlimParser records which page each HTML id attribute lands
on, serializing the map into the .bin cache file.
- Anchor resolution in `EpubReaderActivity`: When navigating to a
footnote link with a fragment (e.g., `chapter2.xhtml#note1`), the reader
resolves the anchor to a page number and jumps directly to it.
- Section file format change: Bumped to version 15, adds anchor map
offset in header.

  ---

## Simplified scope vs. PR #1143

To minimize conflicts and complexity, this PR differs from #1143 in key
ways:

* **Anchors tracked**
  * **Origin:** Only TOC anchors (passed via `std::set`)
  * **This branch:** All `id` attributes

* **Page breaks**
  * **Origin**: Forces new page at TOC chapter boundaries
  * **This branch:** None — natural flow

* **TOC integration**
  * **Origin**: `tocBoundaries`, `getTocIndexForPage()`, chapter skip
  * **This branch:** None — just footnote links

* **Bug fix**
  * **This branch:** Fixed anchor page off-by-1/2 bug


The anchor recording bug (recording page number before `makePages()`
flushes previous block) was identified and fixed during this extraction.
The fix uses a deferred `pendingAnchorId` pattern that records the
anchor after page completion.

  ---

## Positioning for future merge

Changes are structured to minimize conflicts when #1143 eventually
merges:

- `ChapterHtmlSlimParser.cpp` `startElement()`: Both branches rewrite
the same if `(!idAttr.empty())` block. The merged version will combine
both approaches (TOC anchors get page breaks + immediate recording;
footnote anchors get deferred recording).
- `EpubReaderActivity.cpp` `render()`: The `pendingAnchor` resolution
block is positioned at the exact same insertion point where #1143 places
its `pendingTocIndex` block (line 596, right after `nextPageNumber`
assignment). During merge, both blocks will sit side-by-side.
   
  ---

## Why merge separately?

1. Immediate user value: Footnote navigation works now without waiting
for the full TOC overhaul
   2. Easier review: ~100 lines vs. 500+ lines in #1143 
3. Bug fix included: The page recording bug is fixed here and will carry
into #1143
4. Minimal conflicts: Structured for clean merge — both PRs touch the
same files but in complementary ways
---

### AI Usage

Did you use AI tools to help write this code? _**< YES >**_ Done by
Claude Opus 4.6

src/activities/reader/EpubReaderActivity.cpp

@@ -595,6 +595,16 @@ void EpubReaderActivity::render(RenderLock&& lock) {
       section->currentPage = nextPageNumber;
     }
 
+    if (!pendingAnchor.empty()) {
+      if (const auto page = section->getPageForAnchor(pendingAnchor)) {
+        section->currentPage = *page;
+        LOG_DBG("ERS", "Resolved anchor '%s' to page %d", pendingAnchor.c_str(), *page);
+      } else {
+        LOG_DBG("ERS", "Anchor '%s' not found in section %d", pendingAnchor.c_str(), currentSpineIndex);
+      }
+      pendingAnchor.clear();
+    }
+
     // handles changes in reader settings and reset to approximate position based on cached progress
     if (cachedChapterTotalPageCount > 0) {
       // only goes to relative position if spine index matches cached value
@@ -790,12 +800,18 @@ void EpubReaderActivity::navigateToHref(const std::string& hrefStr, const bool s
     LOG_DBG("ERS", "Saved position [%d]: spine %d, page %d", footnoteDepth, currentSpineIndex, section->currentPage);
   }
 
+  // Extract fragment anchor (e.g. "#note1" or "chapter2.xhtml#note1")
+  std::string anchor;
+  const auto hashPos = hrefStr.find('#');
+  if (hashPos != std::string::npos && hashPos + 1 < hrefStr.size()) {
+    anchor = hrefStr.substr(hashPos + 1);
+  }
+
   // Check for same-file anchor reference (#anchor only)
   bool sameFile = !hrefStr.empty() && hrefStr[0] == '#';
 
   int targetSpineIndex;
   if (sameFile) {
-    // Same file — navigate to page 0 of current spine item
     targetSpineIndex = currentSpineIndex;
   } else {
     targetSpineIndex = epub->resolveHrefToSpineIndex(hrefStr);
@@ -809,6 +825,7 @@ void EpubReaderActivity::navigateToHref(const std::string& hrefStr, const bool s
 
   {
     RenderLock lock(*this);
+    pendingAnchor = std::move(anchor);
     currentSpineIndex = targetSpineIndex;
     nextPageNumber = 0;
     section.reset();