Rename Arch -> Ledger references (dependent side; deferred until Microplex settles)

[MaxGhenis]

Rename Arch → Ledger references (microplex-us, dependent side)

Companion to the product-rename tracking issue: PolicyEngine/arch-data#49.

Deferred until Microplex settles/launches. main is an active area (+42
commits, in-flight codex/* branches), so do this as a dedicated PR once the
current work lands — or have the Codex effort own it. Whole-word /
identifier-aware only; never substring (architecture, hierarchical,
search contain "arch").

microplex-us surface to rename

• import arch / from arch ... → ledger — only after arch-data ships the ledger package + an arch re-export shim (see arch-data#49 stage 1).
• src/microplex_us/targets/arch.py → src/microplex_us/targets/ledger.py (+ tests/targets/test_arch.py, test_arch_facts.py).
• console scripts in pyproject.toml: microplex-us-arch-target-* → microplex-us-ledger-target-* (coverage/gaps/parity/refresh/smoke).
• internal refs in pipelines/us.py, performance.py, pe_us_data_rebuild_checkpoint.py, pe_us_recalibrate_from_checkpoint.py.
• prose/docs: "Arch" → "Ledger" (e.g. docs/arch-target-gap-queue.md).
• 49 artifacts/arch_* dirs → ledger_* (regenerable build outputs; copy + update any manifest/registry references, or regenerate).
• verify no substring corruption afterward (exclude architecture, hierarchical, search).

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