Rename Arch -> Ledger (full product rename; deferred until Microplex settles)

[MaxGhenis]

Rename the Arch product → Ledger (full rename: all repos + data + bucket)

Tracking issue for renaming the Arch product to Ledger everywhere.

Deferred: execute when Microplex settles/launches. Both arch-data and
microplex-us are live areas with in-flight work, so stage 0 is opening a
freeze/coordination window before any rename lands.

Scope / product surface

• Repo: PolicyEngine/arch-data → PolicyEngine/ledger-data (now in the PolicyEngine org; the local clone also moves ~/CosilicoAI/arch → ~/PolicyEngine/ledger-data).
• Python package: cosilico-arch (module arch, plus db/micro/calibration) → cosilico-ledger (module arch → ledger).
• R2 bucket: arch (production CPS/ACS/SOI microdata + calibration targets) → ledger.
• Dependents: microplex, microplex-us (companion issue), cosilico.ai, docs.
• Artifacts: 49 artifacts/arch_* dirs in microplex-us.

Principles

• Whole-word / identifier-aware rename only — never a substring replace (search, architecture, hierarchical, march all contain "arch").
• Per-repo PRs, branched from main; each stage non-breaking (backward-compat aliases) and reversible.
• Data = copy-then-cutover, never rename/delete production data in place.

Stages

• 0. Decide canonical names (repo ledger-data, package cosilico-ledger, module ledger, bucket ledger); open a coordination/freeze window with in-flight Codex work.
• 1. arch-data: introduce ledger package/module as canonical with a thin arch re-export shim so importers keep working; rename code/docs whole-word; rename the repo arch-data → ledger-data; move local clone to ~/PolicyEngine/ledger-data.
• 2. Dependents switch import arch → import ledger (after the shim exists): microplex, microplex-us (companion issue), cosilico.ai.
• 3. R2 bucket: copy arch → ledger, enable dual-read, repoint writes, verify, then deprecate arch.
• 4. Artifact dirs: rename the 49 arch_* dirs via copy + reference update, or regenerate (they are build outputs).
• 5. Remove the compatibility shims; deprecate the old repo + bucket.

Full plan: microplex-us rebuild/docs/arch-to-ledger-migration-plan.md (branch claude/scoreboard-first-rebuild).