P3: depreciation straight-line by days through the final month, schedules by day (defects 22, 9, D-37)

The final month takes what the months before leave, spread over its days;
a disposed asset stops on its disposal day. A skipped month is caught up
in the month the run is made; a reversed run un-posts its month. New
schedules spread by day; equal-months schedules must start on the 1st.
Amounts round to the currency's own minor unit. Kani and Lean proofs; an
exhaustive search over runs, reversals and disposals.

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

crates/ledger-core/src/error.rs

@@ -419,10 +419,28 @@ pub enum LedgerError {
     #[error("asset account links must be distinct; account id {0} is linked twice")]
     DuplicateAssetAccount(u64),
 
-    /// purchase_date + useful life overflows the date range.
+    /// purchase_date + useful life overflows the date range: a new asset's
+    /// last day of depreciation is 9999-12-31 at the latest.
     #[error("purchase date plus useful life overflows the calendar")]
     AssetDateOverflow,
 
+    /// A depreciation run for a month that can't be run: not a month, or
+    /// one ending before the asset was placed in service.
+    #[error("invalid depreciation period: {0}")]
+    InvalidDepreciationPeriod(String),
+
+    /// An entry of a fixed asset's that can't be reversed (its disposal,
+    /// or depreciation the disposal took as final), with why.
+    #[error("entry {0} can't be reversed: {1}")]
+    AssetEntryIrreversible(u64, String),
+
+    /// A disposal refused because a depreciation run of the asset carries
+    /// at the end of the disposal's month other than what it carries now
+    /// (a run of a later month that stands; a reversal or re-instatement
+    /// dated after the month, not undone), with what to do.
+    #[error("the disposal is dated before depreciation the books carry: {0}")]
+    DepreciationPastDisposal(String),
+
     /// An amortization schedule or recurring entry is invalid, unknown, or
     /// already posted for that date.
     #[error("invalid schedule: {0}")]