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/settings.rs

@@ -153,6 +153,7 @@ const PARTY_POLICY: &[(&str, &[&str])] =
 const APPROVAL_MODE: &[(&str, &[&str])] = &[("off", &[]), ("enforce", &[])];
 const TAX_CATEGORY_POLICY: &[(&str, &[&str])] =
     &[("warn", &[]), ("refuse", &["required", "require"])];
+const SCHEDULE_CONVENTION: &[(&str, &[&str])] = &[("daily", &[]), ("monthly_even", &[])];
 
 /// A return's key as a setting keeps it.
 pub fn tax_form_key(f: TaxForm) -> &'static str {
@@ -334,6 +335,13 @@ pub static KEYS: &[Key] = &[
         sensitivity: Sensitivity::Public,
         doc: "Prefix of generated asset tags (\"AC\" for AC-0001): one to eight letters or digits. Unset: the company name's initials.",
     },
+    Key {
+        name: "books.schedule_convention",
+        toml: ("books", "schedule_convention"),
+        kind: Kind::Choice(SCHEDULE_CONVENTION),
+        sensitivity: Sensitivity::Public,
+        doc: "How a new prepaid-expense or deferred-revenue schedule spreads its total: daily (from its start date: full months equal, the first and last partial months by their days, like depreciation) or monthly_even (equal months; the schedule starts on the 1st of a month). Unset: daily (D-11). Existing schedules keep theirs.",
+    },
     Key {
         name: "policy.evidence.threshold",
         toml: ("books", "receipt_threshold"),
@@ -1296,6 +1304,14 @@ impl Settings {
         self.str("books.asset_tag_prefix")
     }
 
+    /// `books.schedule_convention`: how a new schedule spreads its total.
+    /// Unset: daily (plan/served.md D-11, confirmed in D-37).
+    pub fn schedule_convention(&self) -> crate::ScheduleConvention {
+        self.str("books.schedule_convention")
+            .and_then(crate::ScheduleConvention::parse)
+            .unwrap_or(crate::ScheduleConvention::Daily)
+    }
+
     /// `policy.parties`.
     pub fn parties(&self) -> PartyPolicy {
         self.str("policy.parties")