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

docs/spec/rules.md

@@ -1757,7 +1757,11 @@ event type would be refused. P6's is `attachment_text` (an
 `attachment.text_extracted`, `attachment.annotated` or `entry.settles`, a
 new event type, and an `attachment.withdrawn` carrying `duplicate_of`; an
 older version would refuse the types, or withdraw a duplicate without
-saying what it duplicates, B-44). A command asks for
+saying what it duplicates, B-44). P4's is `drafts_v2` (an
+`entry.draft_state_set` or `entry.draft_discarded`, both new event types:
+an older version would refuse them, or, reading around them, keep a
+discarded draft that holds a period open and call a waiting draft one
+being built, B-43). A command asks for
 the feature it needs (`Store::ensure_feature`) before its first journaled
 line, and only when the line it is about to write uses it, so a write
 refused for waiting on the upgrade journals nothing, never an entry