Discard unreadable books when the host starts a draft over #573

Part of Phase 4 (#427). Needs 4a (#453). Blocks 4b (#454).

Background

A draft book whose stored analysis can't be read (a torn write, or outside damage) is kept as stored and never written over (#475).

On the WebView write path:

• An edit that reaches the book stays on screen, but writes leave that book out.
• Save to an existing project is refused. Save As leaves the book out.
• New and Open discard the book. This is how the user recovers.

That discard lives in whole-draft saveDraft, through replaceAll's discardUnreadable option. 4c deletes both.

On the host path:

• apply refuses any edit that reaches such a book (Add apply with inverse capture and per-session undo #486).
• A discard keeps the book's stored analysis and shows the book empty (Add catalog edits, discards and removal previews #524).
• Opening a session only loads; it never discards. This matches main, which discards only on saves that start the draft over.
• 4a brings New and Open to the host, but doesn't cover this case.

So on the host path nothing gets the user out yet.

Scope

• Host New and Open discard each unreadable book, so later writes treat it like any other book.
  • In DraftRepository today, this is clearing unreadableShards before the write.
• Do it in whatever path 4a gives New and Open, not through replaceAll.
• Opening a session stays load-only.

Why it blocks 4b

4b ships the host path by default. Without this issue, a user with an unreadable book has no way out on the default path, short of the rollback release.

Done when

• With the flag on, New or Open on a draft with an unreadable book leaves no book unreadable, and edits that reach the book apply.
• Tests cover this on the flag-on path.

Then

• Remove "Books whose stored analysis can't be read" from 4c's body.