feat(us-tax): filing-status-aware capital loss limitation — MFS is $1,500, not $3,000

[elasticdotventures]

Implements #148

• FilingStatus enum with capital_loss_limit()
• MFS returns $1,500 per IRC §1211(b)
• Carryforward chains across years per §1212
• Non-business bad debt = short-term per §166(d)
• Warning when carryforward exceeds 20 years of absorption