Refactor GainsTax function in calcfunctions.py module

[martinholmer]

Refactor GainsTax: collapse hasqdivltcg flag, drop dead c05100, rename c24510

Three behavior-preserving cleanups in GainsTax:

• Replace the 9-line hasqdivltcg int-flag pattern (three if-blocks
  setting 1/0 followed by if hasqdivltcg == 1) with a single boolean
  has_qdivltcg = (not CG_nodiff) and (any of the five qual-div / LTCG inputs > 0).

• Remove dead intermediate local c05100 (= c24580; pure local, never
  returned, never re-read). Foreign-earned-income-exclusion rationale
  moved to a one-line comment; final assembly is now taxbc = c24580.

• Rename single-use local c24510 → line9_base. c24510 was a relic of
  an old Form 1040 Schedule D line label that no longer matches any
  current IRS line; line9_base signals its role as the input to the
  Sch D TW line-9 dwks9 calculation.

No function signature change, no return-tuple change, no body-formula change.

Sch D TW / QDCGTW logic, banner section comments, and docstring unchanged.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (b90305d) to head (6dff19c).

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           13        13           
  Lines         2721      2721           
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  Hits          2721      2721           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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