Fix skipped integer allocation with nonlinear commissions

[Jo2234]

Summary

• preserve the closest affordable integer quantity when the allocation root search skips over it under nonlinear commissions
• add a regression test using the minimum/per-share/capped commission model reported in commission fn is not smooth #461

Reproduction

With a $286 allocation at a $1 price while rebalancing from a short position, the search moved from 286 shares to 284 even though 285 shares cost $285.9975 and were affordable. It then raised the commission fn is not smooth RuntimeError. The corrected search steps back to 285 shares and completes the allocation.

Tests

• python -m pytest -q — 183 passed
• make lint — passed

Fixes #384
Addresses #461