feat(drafts): a draft has a state and can be discarded (P4)

Accounting reason: an entry that is finished but waits on its receipt is
a different thing from one still being built, and a draft that will
never post must have a way out, or the period that holds it open can
never close (the Lean hypothesis hdrafts). JournalEntry gains
DraftState { Building, AwaitingEvidence { note } }; Ledger gains
set_draft_state (a note: no line, balance or report moves),
discard_draft (a draft only; a posted entry is never removed, B-8; ids
are never reused; what was recorded about the draft goes with it) and
posted_seq (the journal line that posted an entry, for the digest's
since_seq). Lean Step.discard with its preservation proof and three
theorems (a discard moves no figure, removes only its draft, and
unblocks a close), listed in Audit.lean.

Tests: discard_unblocks_close (ledger-core invariants).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HS8WZziDnyxunC23MUxrLp

verify/lean/Audit.lean

@@ -101,3 +101,6 @@ open Wanderbooks
 #print axioms Documents.Reachable.control_is_documents
 #print axioms Documents.Step.writeOff_unattached
 #print axioms Documents.writeOff_within_bounds
+#print axioms Step.discard_keeps_books
+#print axioms Step.discard_drafts
+#print axioms Step.discard_unblocks_close