feat: prepaid/deferred amortization schedules and recurring entries

Amortization schedules spread a prepaid expense or deferred revenue over
the months it covers, to the cent, posting on month ends; runs preview
first, are idempotent, honor closed periods, and are exempt from the
receipt policy like depreciation. Recurring entries generate drafts only,
never posts. Six new tools (59 total); four new journal events, replayed
and checked.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MtYEaR6M6Ybdhm6X98BWNu

crates/ledger-core/src/error.rs

@@ -132,6 +132,11 @@ pub enum LedgerError {
     #[error("purchase date plus useful life overflows the calendar")]
     AssetDateOverflow,
 
+    /// An amortization schedule or recurring entry is invalid, unknown, or
+    /// already posted for that date.
+    #[error("invalid schedule: {0}")]
+    InvalidSchedule(String),
+
     /// A tax category doesn't exist, doesn't fit the account, or has a bad
     /// deductible percent.
     #[error("invalid tax category: {0}")]