P1: the books keep how they are kept

Settings, conventions (posting templates, approval rules, policies,
close steps, calendar, mappings, notes), decisions, open items and
entry approvals are journal events (journal format 2). Books that keep
their settings never read wanderbooks.toml or config.json; the server
and the viewer take the vault key from --vault-key-file instead.
books.import_settings moves a settings file in once, exactly as the
server reads it. Approval rules, when enforced, gate every posting
path. Replay refuses what it can't read as newer, never as corrupt.

Lean: a record changes no figure. Mutation on the whole diff: every
survivor killed by a test but two equivalent (docs/testing.md). The
Company page says a late open item is overdue in words, not colour
alone.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-core/src/error.rs

@@ -32,7 +32,9 @@ pub enum LedgerError {
 
     /// A line larger than one quadrillion, or with more than ten decimal
     /// places, is a mistake, and one that could overflow exact arithmetic.
-    #[error("journal line amount is out of range (at most 1000000000000000, ten decimal places)")]
+    #[error(
+        "journal line amount is out of range (at most one quadrillion, with ten decimal places)"
+    )]
     AmountOutOfRange,
 
     /// A date outside the calendar the books keep (`crate::calendar`):
@@ -44,7 +46,7 @@ pub enum LedgerError {
     /// payroll figure, a statement balance, a share, a basis) larger than
     /// one quadrillion or with more than ten decimal places: a mistake, and
     /// one that could overflow exact arithmetic.
-    #[error("{0} is out of range (at most 1000000000000000, ten decimal places)")]
+    #[error("{0} is out of range (at most one quadrillion, with ten decimal places)")]
     ValueOutOfRange(String),
 
     /// A figure the books derive (a sum, or a share of one) too large for
@@ -509,4 +511,36 @@ pub enum LedgerError {
     /// The referenced class does not exist.
     #[error("unknown class id {0}")]
     UnknownClass(u32),
+
+    /// A setting change the books can't honor (crate::settings): an
+    /// unknown key, a value its key refuses, or settings that wouldn't
+    /// hold together.
+    #[error("{0}")]
+    InvalidSetting(String),
+
+    /// A convention, decision, open item or approval the books can't
+    /// take (crate::conventions).
+    #[error("{0}")]
+    InvalidConvention(String),
+
+    /// Approval rules are enforced (`policy.approval_rules = enforce`) and
+    /// rules cover this entry that its approvers haven't approved as it
+    /// stands.
+    #[error(
+        "entry {entry} needs approval before it posts: {missing}; record each with books.approve_entry"
+    )]
+    ApprovalRequired {
+        /// The draft.
+        entry: u64,
+        /// Each rule and its approver, e.g. `over-500 (approver: Pat)`.
+        missing: String,
+    },
+
+    /// Approval rules are enforced, and the entry an operation would post
+    /// at once (an invoice, a bill, a payment, payroll, a disposal) is one
+    /// a rule covers: it has no draft to approve.
+    #[error(
+        "approval rules cover the entry this would post at once ({0}): draft it instead (books.draft_entry), have it approved (books.approve_entry), then post it"
+    )]
+    ApprovalNeedsDraft(String),
 }