fix: preserve payment identity in first annuity-due period

[kadyrbekovhamit-cyber]

Summary

• return the full level payment as principal in the first annuity-due period
• apply the same correction to the vectorized path
• cover scalar pmt = ipmt + ppmt, vectorized ppmt, and full-term cumprinc

Problem

ipmt correctly returns zero interest for period 1 when payment is at the beginning, but ppmt still applies the ordinary annuity-due formula. This breaks the documented payment identity and leaves a residual principal balance.

rate = 0.005, nper = 12, pv = 1000, pmt_at_beginning = true
pmt(…)          = -85.63823851450982
ipmt(…, per=1)  =   0.0
ppmt(…, per=1)  = -80.66311413640034  (before)

The first payment happens at time zero, so it carries no interest and its entire amount is principal. The corrected ppmt is therefore pmt(...).

Validation

• full cargo test --release: 238 passed
• cargo test --release --features nonumpy: 227 passed, 10 ignored
• cargo fmt --check
• mutation check: removing the two new branches fails the regression test with -80.66311413640034 !~= -85.63823851450982