fix(drafts): list drafts as messages, not threadssignatures

The Drafts filter shipped threaded in item 138, reasoning that a draft
reply belongs with the conversation it answers. That reasoning cost the
feature: a thread row stands for its first matched message, which for a
draft reply is the message being replied to, so the draft itself had no
row of its own and double-clicking the conversation opened nothing.

Reversed with the user. Drafts now follows Sent; Trash deliberately does
not, since a deleted message still belongs to its conversation and
nothing there has to be reachable for editing.

The view mode was decided in three places that each compared against
"sent" and had to agree: builtinFilter(), the reader that reapplies the
mode, and the writer that skips storing what the generator implies.
generatorIsFlat() is now the one closed set they share, and
builtinFilter() sets flat from it rather than inside a branch so the set
cannot drift from the labels.

Setting only the branch would have looked correct. Its save/load pair
survives by accident, because the writer's skip knew only "sent" and so
would have stored the key for drafts. The gap is the reader's fallback,
for a file carrying no flat key at all: an older build, a migration or a
hand edit comes back threaded against a flat button, and the next save
persists the disagreement.

theDraftsFilterIsThreadedNotFlat is inverted rather than deleted, keeping
its history, and now also pins Trash as threaded. The round trip is
covered by extending aGeneratedEntryWritesNoRedundantKeys, which already
asserted that property for Sent. Mutation-checked: reverting
generatorIsFlat() to "sent" alone fails both.

Suite 37 of 38; undoMovesTheMessageBack is item 136, pre-existing and on
an unrelated path.

Closes item 159.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UUQS6n3cmsFrsjCNmwNtf8

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