diff --git a/crates/ledger-core/src/lib.rs b/crates/ledger-core/src/lib.rs
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+++ b/crates/ledger-core/src/lib.rs
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+//! `ledger-core`: canonical double-entry bookkeeping semantics.
+//!
+//! This crate is the engine. Everything else in the workspace — persistence,
+//! import, the oracle harness, the UI — builds on top of it and never
+//! reimplements accounting logic.
+//!
+//! Design rules (see `AGENTS.md` at the repo root):
+//!
+//! * Money is [`Money`]: an exact [`rust_decimal::Decimal`] plus an ISO 4217
+//!   currency code. Binary floats never touch money paths.
+//! * A [`Ledger`] is single-currency for now (see `docs/requirements.md` R-5).
+//! * All mutations go through [`Ledger`] commands (`create_entry`, `add_line`,
+//!   `post_entry`, `reverse_entry`). Posted entries are immutable; corrections
+//!   are reversing entries.
+//! * Reports ([`trial_balance`], [`balance_sheet`], [`profit_and_loss`]) are
+//!   pure functions of a ledger, which is what makes the semantic oracle
+//!   (`docs/oracle.md`) possible.
+
+pub mod account;
+pub mod error;
+pub mod journal;
+pub mod ledger;
+pub mod money;
+pub mod reports;
+
+pub use account::{Account, AccountId, AccountType, ChartOfAccounts, Side};
+pub use error::LedgerError;
+pub use journal::{EntryId, EntryStatus, JournalEntry, JournalLine};
+pub use ledger::Ledger;
+pub use money::{Currency, Money};
+pub use reports::{BalanceSheet, ProfitAndLoss, TrialBalance, TrialBalanceLine};