🐛 Fix need role in section headings by registering NeedRef node

[Copilot]

Using :need: in an RST section heading crashes the build because SphinxContentsFilter (used by TocTreeCollector for TOC generation) doesn't know how to visit NeedRef nodes.

.. req:: My Requirement
   :id: REQ_001

Section with need ref :need:`REQ_001`
-------------------------------------

Fails with: SphinxContentsFilter visiting unknown node type: NeedRef

Fix: Register NeedRef with app.add_node() using dummy visitors, same pattern as NeedPart:

app.add_node(
    NeedRef,
    html=(visitor_dummy, visitor_dummy),
    latex=(visitor_dummy, visitor_dummy),
)

• sphinx_needs/needs.py — add node registration for NeedRef
• tests/test_role_need_in_heading.py — new test asserting the heading renders with a resolved need reference
• tests/doc_test/doc_role_need_in_heading/ — minimal test project

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.30%. Comparing base (4e10030) to head (0c96117).
⚠️ Report is 277 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1700      +/-   ##
==========================================
+ Coverage   86.87%   89.30%   +2.42%     
==========================================
  Files          56       72      +16     
  Lines        6532    10325    +3793     
==========================================
+ Hits         5675     9221    +3546     
- Misses        857     1104     +247     

Flag	Coverage Δ
pytests	89.30% <100.00%> (+2.42%)	⬆️

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