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

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

@@ -133,11 +133,21 @@ void ChapterHtmlSlimParser::startNewTextBlock(const BlockStyle& blockStyle) {
       // This handles cases like <div style="margin-bottom:2em"><h1>text</h1></div> where the
       // div's margin should be preserved, even though it has no direct text content.
       currentTextBlock->setBlockStyle(currentTextBlock->getBlockStyle().getCombinedBlockStyle(blockStyle));
+
+      if (!pendingAnchorId.empty()) {
+        anchorData.push_back({std::move(pendingAnchorId), static_cast<uint16_t>(completedPageCount)});
+        pendingAnchorId.clear();
+      }
       return;
     }
 
     makePages();
   }
+  // Record deferred anchor after previous block is flushed
+  if (!pendingAnchorId.empty()) {
+    anchorData.push_back({std::move(pendingAnchorId), static_cast<uint16_t>(completedPageCount)});
+    pendingAnchorId.clear();
+  }
   currentTextBlock.reset(new ParsedText(extraParagraphSpacing, hyphenationEnabled, blockStyle));
   wordsExtractedInBlock = 0;
 }
@@ -151,7 +161,7 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
     return;
   }
 
-  // Extract class and style attributes for CSS processing
+  // Extract class, style, and id attributes
   std::string classAttr;
   std::string styleAttr;
   if (atts != nullptr) {
@@ -160,6 +170,9 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
         classAttr = atts[i + 1];
       } else if (strcmp(atts[i], "style") == 0) {
         styleAttr = atts[i + 1];
+      } else if (strcmp(atts[i], "id") == 0) {
+        // Defer recording until startNewTextBlock, after previous block is flushed to pages
+        self->pendingAnchorId = atts[i + 1];
       }
     }
   }
@@ -374,6 +387,7 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
                 if (self->currentPage && !self->currentPage->elements.empty() &&
                     (self->currentPageNextY + displayHeight > self->viewportHeight)) {
                   self->completePageFn(std::move(self->currentPage));
+                  self->completedPageCount++;
                   self->currentPage.reset(new Page());
                   if (!self->currentPage) {
                     LOG_ERR("EHP", "Failed to create new page");
@@ -990,7 +1004,12 @@ bool ChapterHtmlSlimParser::parseAndBuildPages() {
   // Process last page if there is still text
   if (currentTextBlock) {
     makePages();
+    if (!pendingAnchorId.empty()) {
+      anchorData.push_back({std::move(pendingAnchorId), static_cast<uint16_t>(completedPageCount)});
+      pendingAnchorId.clear();
+    }
     completePageFn(std::move(currentPage));
+    completedPageCount++;
     currentPage.reset();
     currentTextBlock.reset();
   }
@@ -1003,6 +1022,7 @@ void ChapterHtmlSlimParser::addLineToPage(std::shared_ptr<TextBlock> line) {
 
   if (currentPageNextY + lineHeight > viewportHeight) {
     completePageFn(std::move(currentPage));
+    completedPageCount++;
     currentPage.reset(new Page());
     currentPageNextY = 0;
   }