Refactor TaxInc function to mirror Form 8995/8995-A line structure

[martinholmer]

Adds banner section comments mirroring Form 8995 / Form 8995-A (2025):
pre-QBID taxinc (line 11/33) → QBI build (line 1c/2) → Parts II/III
W-2/UBIA cap and SSTB phase-in (with the four sub-cases
SSTB-above-upper, non-SSTB-above-upper, non-SSTB-in-phase-in, and
SSTB-in-phase-in labeled and annotated against Sch A's applicable %
and Part III line 26) → Part IV income limitation (line 36/14) →
reform-only QBID phase-out → reform-only minimum-floor → Form 1040
line 15. Per-line annotations added (Form 8995 line 5 / 8995-A line 3,
line 21 lower threshold, line 23 gap, lines 5/9/10 wage/alt/full cap,
line 12/34 net_cg, line 14/36 income cap).

Renames local pre_qbid_taxinc_gap to gap and hoists gap /
upper_thd once. Inline-labels each odeds summand with its Sch 1-A
Part. Docstring rewritten to cite Form 8995 / Form 8995-A (2025) and
Form 1040 lines 13-15, document the simplified-vs-full threshold
dichotomy and the four phase-in sub-cases, list QBI components and the
wage/investment exclusion, flag reform-only switches (PT_qbid_limited,
PT_qbid_ps/_prt, PT_qbid_min_qbi/_min_ded), document the Sch 1-A odeds
inclusion in pre-QBID taxinc, and tie each parameter to its form
line. Stale "2018 Pub 535 Worksheet 12-A" comments dropped.

Signature, parameter ordering, and return tuple unchanged.
No formula changes; behavior preserved.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (8500f12) to head (0cb5a49).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #3041   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines         2721      2721           
=========================================
  Hits          2721      2721           

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

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.