enhance: optimize manifest parsing

[anidixit64]

title: "Optimize Parsing EPUB Manifest"

Acceptance Criteria

A checklist of conditions that must be met for this feature to be considered complete.

• In the parse_epub method, before the for itemref in itemrefs: loop, create an empty dictionary: manifest_items = {}
• Find the manifest element and iterate through its items once, populating the dictionary with id as the key and href as the value
• Inside the for itemref in itemrefs: loop, replace the XML search with a simple dictionary lookup

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Definition of Done

• Feature implemented as per Acceptance Criteria
• All new code is covered by pytest tests
• Ensure >90% coverage on the changed or new code
• Documentation (docstrings and Sphinx docs) is updated
• The pre-commit hooks pass on all changed files
• [ ]