Change number field signature display from [r1, r2] to (r1, r2)

[Copilot]

Number field signatures are currently displayed with square brackets but should use parentheses to match standard mathematical notation.

Changes

• web_number_field.py: Added signature_display() method returning formatted string '(r1, r2)'. The existing signature() method remains unchanged, returning [r1, r2] list for internal use by is_real_quadratic(), unit_rank(), cnf(), and other methods that depend on list indexing.

• number_field.py: Updated field page rendering (line 468) and search column formatting (line 853) to use signature_display().

• test_numberfield.py: Added test_signature_display() to verify parentheses format on field detail pages.

Example

# Before
nf.signature()  # Returns [2, 1] - still works for internal code
# Display showed: [2, 1]

# After  
nf.signature()  # Returns [2, 1] - unchanged
nf.signature_display()  # Returns '(2, 1)' - used for display
# Display shows: (2, 1)

Note: Search URLs continue using [r1,r2] format for parser compatibility. Data downloads use list format for machine readability.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Signature display for number fields should be use brackets instead of square brackets</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Signature display for number fields should be use brackets instead of square brackets #6840

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[AndrewVSutherland]

I think @jwj61 should review this before it is merged

[jwj61]

Will look at when tests pass