feat(store): draft states and discards are journaled, posted_seq is kept (P4)

Accounting reason: a draft that waits on its receipt, and one thrown
away, are facts about the books' working papers that must survive a
restart and replay identically, or a discarded draft would come back and
hold its period open. entry.draft_state_set and entry.draft_discarded are
drafts_v2's (an older version stops at the declaration instead of
misreading them); no figure moves. Schema 19 (taken at merge as the next
after the current last): entries.posted_seq, and a draft_states table
(drafts are not rows of entries, so the plan's entries.draft_state column
has nowhere to live). posted_seq is the journal line that posted each
entry (entry.posted, entry.reversed, the line that closed a period or
posted a schedule month), identical live and on replay.

8.0.6: a hand-written line of each new shape in
books_written_under_older_rules_still_open; fixture old-books/p4_before
written by 25b52580 (hand entries posted late, a reversal, a document, a
schedule month, a recurring draft, open drafts, a close) opens to the
same figures (serialized entries omit a building state, so no golden
moved); drafts_v2_is_its_own_feature; the fuzzer and GUI journal know the
types.

Tests: draft_states_replay_and_project, drafts_v2_waits_for_the_upgrade,
posted_seq_is_the_posting_lines_seq, an_older_version_stops_at_the_draft_events.

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

crates/ledger-mcp/handbook/format-upgrade.md

@@ -8,4 +8,4 @@
 - After the upgrade an older version of wanderbooks can't open these books; the backup made first keeps them as they were. Going back is the owner's choice, never yours.
 - This upgrade is `documents_v2`: invoice numbers, terms and line accounts (S4), sent records (S5), payments that settle several documents with early-payment discounts and a processor's fee (S6), and write-offs (S7). Until it is given, those writes are refused and everything else works as before.
 
-- The journal features an upgrade can add: `members_cumulative`, `account_renamed`, `require_tax_category`, `tax_categories_gain_exempt`, `disposal_before_service`, `depreciation_catch_up`, `party_creditors`, `payment_instruments`, `vendor_1099_decisions`, `schedule_convention`, `month_reposting`, `asset_basis`, `projects`, `document_line_projects`, `document_standing`, `documents_v2`, and `attachment_text` (P6: a filed file's extracted text, what a human annotated of it, and the entries a payment settles). All are declared at once; a version that doesn't know one stops at the declaration instead of misreading the books.
+- The journal features an upgrade can add: `members_cumulative`, `account_renamed`, `require_tax_category`, `tax_categories_gain_exempt`, `disposal_before_service`, `depreciation_catch_up`, `party_creditors`, `payment_instruments`, `vendor_1099_decisions`, `schedule_convention`, `month_reposting`, `asset_basis`, `projects`, `document_line_projects`, `document_standing`, `documents_v2`, `attachment_text` (P6: a filed file's extracted text, what a human annotated of it, and the entries a payment settles), and `drafts_v2` (P4: a draft's state, awaiting its evidence, and a discarded draft). All are declared at once; a version that doesn't know one stops at the declaration instead of misreading the books.