Fix EPUB reading order when navigation documents are outside the spine

[cary-rowen]

Link to issue number:

Fixes #419

Summary of the issue:

Some EPUB 3 books are read in manifest order instead of spine order when the manifest contains an XHTML document that is not referenced by the spine, commonly nav.xhtml. This can cause chapters to appear in the wrong order and navigation content to be included in the book text.

Description of how this pull request fixes the issue:

Builds a manifest ID lookup once and processes documents in spine order with linear complexity.

Non-spine navigation documents are excluded from the reading content, while other auxiliary XHTML documents remain available after the main reading order. Partially invalid spine entries are skipped with a warning, and manifest order remains as a fallback when no usable spine entries exist.

The preprocessed HTML cache version is updated so previously cached books are rebuilt with the corrected order.

Testing performed:

• Loaded the sample EPUB from EPUB sections are read in wrong order: spine sort is silently abandoned when a manifest document is not in the spine (nav.xhtml) #419 and verified that chapters are presented in the order 1, 2, 3, 4, and 5.
• Verified that the navigation document, including its contents and page list, is not included in the reading content.
• Reopened the sample with an existing version 1 content cache and verified that the corrected content is regenerated.
• Checked EPUBs with navigation documents inside the spine and EPUBs with partially invalid spine references. Existing readable content and auxiliary XHTML documents remained available.

Known issues with pull request:

None known.

[cary-rowen]

cc @tamaracpadilha