Tier 2: capital-loss carryforward and NIITVerified

Two cross-border fidelity gaps closed in the tax estimate.

Capital-loss carryforward: US net capital losses beyond the $3,000/yr ordinary
offset no longer vanish. %apply-loss-year advances a carryforward through each
year (losses offset gains first, then up to $3,000 offsets ordinary, the rest
carries on); capital-loss-carryin walks the journal's earlier years so
compute-taxes nets this year's gains against prior losses. New tax-result field
us-cap-loss-cf, shown in the report. (journal-years moved to ledger.lisp so the
tax layer can use it.)

NIIT: the 3.8% Net Investment Income Tax on the lesser of net investment income
(interest, dividends, rents, royalties, capital gains) and MAGI over the
non-indexed threshold ($200k single / $250k mfj). US persons only; it is not
creditable against Canadian tax, so it is flagged as a distinct line and folded
into the US total.

Adds capital-loss-carryforward and niit-net-investment-income-tax tests.
158 checks pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>