standard_ledger 0.3.0

StandardLedger captures the recurring 'append-only entry → N projection updates' pattern as a declarative DSL on host ActiveRecord models. Supports five projection modes — :inline, :async, :sql, :matview, :trigger — plus a deterministic rebuild path from the entry log, ad-hoc materialized view refresh, and a doctor rake task that verifies host-owned trigger presence. Enforces idempotency-by-unique-index and immutability at the entry level.