[satoryu]

Why

Most behavior was only tested through Docx::Document and real .docx fixtures. That works as an end-to-end test, but it means a failure doesn't point at the class that broke, and element behavior can't be tested without crafting a fixture.

What

The suite now mirrors lib/, one spec per class, in three layers:

Layer	Location	How
Unit	spec/docx/containers/*, spec/docx/elements/*, spec/docx/helpers_spec.rb	Nodes come from WordprocessingML fragments (w_node / w_body in spec/support/xml_helpers.rb). document_properties is passed as a plain hash, and the Document is an instance_double.
Document	spec/docx/document_spec.rb	Only what Document owns: opening the package, loading parts (styles, rels, headers, footers), document-wide lookups, and save / stream / replace_entry.
Integration	spec/integration/*	End-to-end scenarios on real fixtures: save-and-reopen round trips, plus regressions that depend on a real file's structure (Google Docs bookmarks, split placeholders, …).

Housekeeping:

• spec_helper now loads docx/document. lib/docx.rb only autoloads Document, so style_spec worked only when it happened to run after document_spec.
• Saved files go to a per-example temp dir (tmp_path) instead of spec/fixtures.
• The no_styles font_size check was inside the 'reading' shared examples and described as "should raise an error" while asserting nil. It moved to Document#font_size.
• Removed an empty example and a duplicated example name.
• Updated the tdd-development skill and CLAUDE.md with where new tests go.

No library code changes.

Verification

• 164 → 266 examples, all green across several random seeds.
• Line coverage went from 96.36% to 98.92%. No line that was covered before is uncovered now.
• Mutation check: I broke 12 representative spots in lib/ (substitute search offset, html alignment/size/strike, cell → paragraph property passing, column indexing, block-bookmark pPr handling, html_tag, style_of lookup, inspect truncation, left alignment). The unit specs alone catch every one.

Bugs found while writing unit specs (not fixed here)

These are behavior changes, so each is tracked as its own issue and will get a test-first PR:

1. Element#prepend_to always raises NoMethodError #189 Element#prepend_to calls element.node.properties, which doesn't exist on Nokogiri nodes, so the method always raises.
2. TextRun#text= on a run without w:t leaves #text stale #190 TextRun#text= on a run with no w:t writes the XML, but #text still returns "" because @text_nodes isn't refreshed.
3. SimpleInspect#inspect(true) raises ArgumentError #191 SimpleInspect#inspect(true) raises ArgumentError: the bare super forwards the argument to Object#inspect.
4. Document without word/_rels/document.xml.rels raises NoMethodError on #paragraphs #192 If word/_rels/document.xml.rels is missing, load_rels rescues and leaves @rels nil, and then #hyperlinks (and so #paragraphs) raises NoMethodError.

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