docs: fold full thread rendering and batch tagging into spec and plan

Both were narrowed in the first draft and are now in scope. Newsletter
threads make whole-thread rendering fundamental, and bulk archive is the
main thing a GUI buys over neomutt.

Two consequences fell out of thread rendering and are recorded in both
documents. The thread renders as one document in one web view rather than
one view per message, since a view each would spawn a Chromium render
process each. Sharing a document makes cid: references collide when two
messages use the same Content-ID, which is common across newsletters from
one sender, so every reference is namespaced to cid:<prefix>!<id>.

Deciding which messages render expanded needs the query, so loadThread now
intersects it with the thread and MessageRef carries a matched flag.

Batch tagging resolves thread ids to message ids on the worker in one
combined query. Undo stores thread ids and re-resolves, staying correct
after the selection moves.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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