Add keyword-only `concurrency: int = 1` to `run_portfolio`, a module-level
`_waves` partitioner, and a fail-fast on k < 1 that precedes any project load.
The execution loop is deliberately NOT changed here. At the default k=1 every
wave holds a single project, so the wave partition reproduces the existing
`for pid in ids` order by construction — the parameter is inert until Step 3
wires the executor. The 5 tests pin exactly that: order preservation across
every k, nothing dropped or duplicated, and the k<1 raise.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LQapztREtC2mkr5oU811pr
