fix(word-addin): give copied documents their own chat identity

WHY THIS MATTERS
The add-in identifies a document by a UUID stored in
Office.context.document.settings -- and Office embeds those settings in
the .docx itself. "Save As" or a file copy therefore carries the UUID
into the copy, which silently inherits the original's ENTIRE chat
history (cloud and local storage are both keyed by this ID) and its
tracked-edit anchor registry. Chats about one contract surface inside a
different file; anchors point at revisions that don't exist there.

WHAT IS THE FAILURE MODE
Document settings are the right place for identity (they survive
renames and moves), but they conflate "same document" with "same file
lineage". A second identity signal is needed to tell a moved original
apart from a spawned copy.

HOW IT WORKS
A companion setting stores the normalized document URL next to the
UUID. On load:
- both stored and current URL known and different  -> this is a copy:
  mint a fresh UUID (createSecureUuid), persist it with the new URL,
  and clear the stale anchor registry -- all batched into one saveAsync;
- stored URL missing (first open after upgrade)    -> keep identity,
  adopt the current URL;
- current URL empty or unavailable (unsaved doc)   -> keep identity,
  store nothing.
Normalization is trim + trailing-slash strip + lowercase, deliberately
without SharePoint URL canonicalization: an over-eager "copy" verdict
would orphan real chat history, so ambiguity always resolves to
keeping the existing identity.

The e2e Office mock gains a seedable document.url; new specs cover the
Save As path (fresh document_id, updated URL setting, anchor registry
cleared) and both keep-identity paths.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015U1iPxsADQ2yuksNmDuvvW