crosspoint-reader-mod

Uri Tauber 4d22256745 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.

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## 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.

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## 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.

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## 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.
   
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## 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
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### AI Usage

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