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

crates/ledger-core/src/error.rs

@@ -854,6 +854,12 @@ pub enum LedgerError {
     #[error("{0}")]
     InvalidVoid(String),
 
+    /// A draft's state or discard was refused: the message says why (a
+    /// note or reason that breaks the books' text rule, no reason given).
+    /// Plan P4, rules.md B-43.
+    #[error("{0}")]
+    InvalidDraft(String),
+
     /// A void invoice can't be recorded as sent: it was never a document
     /// the customer should have received (plan P4b S5, B-38).
     #[error("invoice {0} is void: a void invoice was never sent")]