Use loss-limited net capital gain in Delaware pension exclusion basket

[PavelMakarchuk]

Summary

The Delaware pension exclusion eligible-retirement-income basket
(de_pension_exclusion_income, via
gov.states.de.tax.income.subtractions.exclusions.pension.income_sources)
summed the raw, unlimited capital_gains variable (short-term + long-term
net, with no loss limit). A large capital loss flowed in at full magnitude,
drove each spouse's basket sharply negative, and min(cap, max(0, eligible_income))
in de_pension_exclusion then clamped the whole exclusion to $0 — even for a
60+ filer with far more than $12,500 of otherwise-eligible interest/dividends/pensions.

This PR replaces capital_gains with the person-level, federally loss-limited
variable loss_limited_net_capital_gains_person.

Why raw capital gains is wrong here

Delaware taxable income begins with federal AGI. By IRC § 1211(b) a joint
filer's deductible net capital loss is capped at −$3,000/year (excess carries
forward), so no Delaware PIT-RES return ever reflects a multi-million-dollar loss.

The 2025 Form PIT-RES, Line 6 pension-exclusion worksheet defines eligible
retirement income to include "capital gains net of capital losses" — the figure
as it appears on the return, i.e. the federal Schedule D loss-limited net, not the
raw unlimited sum of transactions. Delaware does not decouple from the § 1211 limit.
A capital loss can therefore reduce the basket by at most $3,000 and cannot wipe out
an otherwise-eligible balance exceeding $12,500.

Statutory / form basis:

• 30 Del. C. § 1106(b)(3) — up to $12,500 exclusion for age 60+ on pensions and
  eligible retirement income (dividends, capital gains, interest, rental income,
  qualified plans).
• 2025 Form PIT-RES, Line 6 worksheet — "capital gains net of capital losses."
• IRC § 1211(b) — −$3,000 net capital loss limit for a joint filer.

PR #8502 (issue #8501) fixed only the bleed of a negative exclusion into other DE
subtractions; it left the raw, unlimited capital_gains in the basket. This PR
addresses that residual.

Impact (originating TAXSIM case, out2psl/916)

DE joint, primary age 75, spouse age 40, TY 2025: ~$85,793 interest, a large net
capital loss, spouse $10,000 wages.

System	DE income tax	Head pension exclusion
PolicyEngine (before)	$3,547.51	$0
PolicyEngine (after)	$2,860.25	$12,500
TAXSIM-35	$2,885.17	$12,500
TaxAct	≈$2,851	$12,500

§ 1211 limits the joint net capital loss to −$3,000, so the basket
(≈$85,793 interest − $3,000) is well above the $12,500 cap and the filer keeps the
full exclusion.

Tests

• Corrected the existing de_pension_exclusion.yaml test that encoded the buggy
  $0 behavior from a raw −$1.4M loss (a loss that cannot appear on a DE return) to
  the loss-limited treatment (full $12,500 exclusion).
• Added an integration test: DE 60+ filer with a large capital loss and interest
  above the cap → de_pension_exclusion: [12_500, 0].
• policyengine-core test .../de_pension_exclusion.yaml -c policyengine_us: 8 passed.

Originating TAXSIM issue: PolicyEngine/policyengine-taxsim#916

Fixes #8836.

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