docs: rewrite v3 spec after full cross-check against v2 principles and Codex review

Fixed: added HexacoProfile to Colonist type, PromotionDecision contract,
pre-filtered candidate shortlists (top 5 per dept), deterministic outcome
classification via riskyOption field in scenarios, child trait inheritance,
session reuse instead of agent recreation, drift rate caps and bounds.
All v2 principles preserved: kernel-owned drift, typed contracts, research
packets, model tiering, seed reproducibility.

Author: jddunn