Control roles in the journal (books.assign_role, books.roles); the MCP reloads wanderbooks.toml on change and reports settings problems (books.settings, digest); new writes a full commented wanderbooks.toml

- core: ControlAccount keys, Ledger::assign_role/check_assign_role/role_account (rules.md B-23): one account per role, of the role's type, balance sheet roles move only when empty; invoices, bills, payroll, remittance, and closes post to the assigned account; an assigned role frees its default code for renumbering
- store: role.assigned journaled and replayed (strict account_id)
- mcp: settings.rs reloads by content at the start of each request, keeps the last good settings on a broken edit; books.settings with chart problems; digest settings_problems; instructions say when to edit the toml
- viewer: journal text for role.assigned
- cli: full example wanderbooks.toml, tested to load with every example uncommented
- tests: spec_roles.rs (25, spec-blind), store roles.rs (6), MCP reload/problems/roles, settings unit tests

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GWVo7x3UHw5kwCUaWxiejX

crates/ledger-core/src/error.rs

@@ -97,8 +97,8 @@ pub enum LedgerError {
     /// decides with the human how to handle it.
     #[error(
         "account {code} \"{name}\" is {}, but {role} needs account {code} to be a postable {expected:?} account; \
-         wanderbooks won't post {role} there. Decide with the books' owner how to handle it \
-         (for instance, renumber that account so {code} is free, if they agree)",
+         wanderbooks won't post {role} there. Decide with the books' owner how to handle it: \
+         assign {role} to an account of theirs, or renumber that account so {code} is free",
         if *placeholder { format!("a placeholder {found:?} account") } else { format!("a {found:?} account") }
     )]
     ControlAccountMismatch {
@@ -116,6 +116,44 @@ pub enum LedgerError {
         placeholder: bool,
     },
 
+    /// No control role has that name (see `ledger_core::control_accounts`).
+    #[error(
+        "no role {0:?}; the roles are receivables, revenue, payables, general_expense, \
+         retained_earnings, salary_expense, payroll_tax_expense, federal_withholding, \
+         state_withholding, social_security_payable, medicare_payable, other_deductions_payable"
+    )]
+    UnknownRole(String),
+
+    /// One account per role: an account already filling a role can't take
+    /// a second (its balance would mix what the two control).
+    #[error("account {code} \"{name}\" already holds {role}; give each role its own account")]
+    RoleTaken {
+        /// The account's code.
+        code: String,
+        /// The account's name.
+        name: String,
+        /// The role it fills, e.g. "accounts receivable".
+        role: String,
+    },
+
+    /// A balance sheet role (receivables, payables, withholdings, retained
+    /// earnings) can't move while its account holds a balance: the control
+    /// account must keep agreeing with what it controls.
+    #[error(
+        "{role} is in account {code} \"{name}\", which holds {balance}; move that balance to the \
+         new account with a journal entry first, then assign the role"
+    )]
+    RoleHoldsBalance {
+        /// The role, e.g. "accounts receivable".
+        role: String,
+        /// The code of the account filling it now.
+        code: String,
+        /// That account's name.
+        name: String,
+        /// Its balance.
+        balance: rust_decimal::Decimal,
+    },
+
     /// A rule set by law (a dollar threshold, a rate) isn't known to
     /// wanderbooks for the year asked; the engine never guesses (see
     /// [`crate::rules`]).