Decouple load_rels from load_styles so @rels loads without styles.xml (#158)

[satoryu]

Summary

Fixes #158. Relationships (and therefore hyperlinks) are now loaded independently of styles, so a document without word/styles.xml no longer leaves @rels uninitialized.

The bug

load_rels was called from inside load_styles:

def load_styles
  @styles_xml = @zip.read('word/styles.xml')   # raises Errno::ENOENT if missing
  @styles = Nokogiri::XML(@styles_xml)
  load_rels                                     # never reached when styles.xml is absent
rescue Errno::ENOENT => e
  warn e.message
  nil
end

When a document had no word/styles.xml, the @zip.read raised Errno::ENOENT, the rescue swallowed it, and load_rels never ran. @rels stayed nil, so any later call to #hyperlinks crashed:

undefined method 'xpath' for nil   # document.rb, #hyperlink_relationships

The fix

• load_rels is now called directly from initialize, independently of load_styles.
• load_rels also rescues a missing rels file gracefully (same pattern as load_styles), preserving the previous "swallow and warn" behavior for documents that genuinely have no rels.

Tests

• New fixture spec/fixtures/no_styles_with_hyperlink.docx — a document that has a hyperlink relationship but no word/styles.xml (derived from formatting.docx with word/styles.xml removed).
• New spec asserts #hyperlinks does not raise and returns {"rId4" => "http://www.google.com/"} for that document.
• Confirmed the spec fails on master (NoMethodError) and passes with this change. Full suite green locally (146 examples, 0 failures).

Closes #158

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