Projects: classes as entities, and the sidebar fits short windows

A project is a class with facts (description, status, kind, party,
dates, links, notes) in one project.set event behind the format
feature projects. Four tools (books.add_project, update_project,
projects, project); documents and decisions link to projects; B-37:
on-spec costs are expensed as incurred, repayment is revenue, and the
unrecovered cost is never below zero (Kani). Viewer: a Projects list
and a project page with its figures, by month and by account. The
sidebar measures its footer instead of reserving a fixed 172 px.

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

crates/ledger-core/src/proofs.rs

@@ -1785,6 +1785,28 @@ fn basis_figures_laws() {
     );
 }
 
+/// `kernel::unrecovered_cost` (rules.md B-37 item 4), for every revenue
+/// and expense a line could carry: never below zero; the shortfall when
+/// expenses exceed revenue, else zero; never above max(expenses, 0) when
+/// revenue isn't negative (expenses may be negative: a reimbursement
+/// credited to the expense above its cost).
+#[kani::proof]
+fn unrecovered_cost_laws() {
+    let (revenue, expenses) = (any_amount(), any_amount());
+    let u = kernel::unrecovered_cost(revenue, expenses).expect("no overflow");
+    assert!(u >= 0);
+    if expenses >= revenue {
+        assert_eq!(u, expenses - revenue);
+    } else {
+        assert_eq!(u, 0);
+    }
+    if revenue >= 0 {
+        assert!(u <= expenses.max(0));
+    }
+    kani::cover!(u > 0, "costs not yet repaid");
+    kani::cover!(u == 0 && revenue > expenses, "repaid, and more");
+}
+
 // ---------------------------------------------------------------------------
 // The checker itself
 // ---------------------------------------------------------------------------