docs: add ledger evaluation path

[ChelseaKR]

Summary

• add a five-minute synthetic walkthrough to the README and docs
• add community-archivist, threat-model, and manual accessibility review packets
• add a release-readiness checklist that distinguishes completed checks from owner- and reviewer-controlled prerequisites

Why

Ledger had a working demo and release workflow, but no short public path from evaluation to adoption readiness. These docs make the existing evidence usable without overstating what it proves.

Validation

• make demo passed (synthetic no-outing proof)
• visually checked the local synthetic browse surface
• make verify passed (1,005 tests plus lint, types, i18n, accessibility, dependency, secret, claims, and workflow checks)

Release note

This PR intentionally does not create or tag v0.1.0. The new checklist keeps the release blocked until PyPI trusted publishing, signed-tag identity, and actual independent security/accessibility review evidence are complete.