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

docs/verification.md

@@ -608,6 +608,28 @@ credit, and the bounds are the existing date-sum's).
       books built from empty); the cash-basis refusal is a policy on the
       books' settings, not arithmetic, and is tested, not modelled.
 
+### Drafts: a discard moves no figure (P4)
+
+A draft's state (`Ledger::set_draft_state`) is a note on the draft and
+changes no line, so the model keeps nothing for it (like a record, it is
+outside the books' figures). `Ledger::discard_draft` removes a draft: a
+new command, so a new Lean step, and no kernel function.
+
+38. **A discard moves no figure and takes only its draft.** `Step.discard`
+    removes one entry from `drafts`; the posted log, the chart, the
+    periods and the records are the very same, so every balance, report
+    and closing read from them is the same. `Inv.draftsOpen` is kept
+    (the drafts left were drafts before), `Step.extends` and
+    `Step.frozen` hold with no new argument, and a period with exactly
+    one draft inside it can close once that draft is discarded
+    (`Step.discard_unblocks_close`, the `hdrafts` hypothesis of
+    `Step.close`).
+    - Lean `Step.discard`, `Step.discard_keeps_books`,
+      `Step.discard_drafts`, `Step.discard_unblocks_close` (in
+      `Audit.lean`) · tests `discard_unblocks_close` (the engine's side:
+      a period with an open draft can't close, discarded it can, and
+      every balance and id is as it was).
+
 ## What the proofs found
 
 The engine already had 190+ tests, property tests, mutation testing,