Add ADR-009: Actor-model NetFlow generation

Proposes a dedicated NetFlow generator using an actor-model simulation
where network hosts execute behavioral sequences that produce correlated
flow records as a byproduct. Covers v5 and v9 wire formats, explains why
the block cache is inapplicable (temporal compression), and follows the
dedicated-generator precedent set by ProcessTree/FileTree/Container.

Draft for human review — no implementation included.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: blt