docs: core wiring audit for microplex primitives

[MaxGhenis]

Summary

Maps microplex core primitives to microplex-us usage. Classifies each as WIRED / READY / PARTIAL / PROTOTYPE / UNKNOWN and identifies decisions that are load-bearing for the spec-based-ecps-rewire path.

Key findings

• Core has ~80% of primitives the rearchitecture needs (period, sources, transitions, calibration, reweighting, fusion, models, hierarchical, geography, validation); most are unused by microplex-us today.
• microplex-us reinvents donor-integration / calibration / entity-table logic that core already provides.
• Project framing shifts from "design + build" to "audit + wire + deprecate".

Critical decision points raised

• Three-way calibration overlap (Calibrator / Reweighter / microcalibrate) — must pick mainline before migration step 2.
• Hardcoded US rates (SSA life tables, CPS rate tables) sit inside generic microplex.transitions/* — either move or make pluggable before UK port.
• US-specific data sources (cps, puf, psid) and validation (soi) live in microplex core and should move to microplex-us.

Test plan

• Doc-only change; no code to test.
• Review each primitive classification against current behavior before committing to migration order.