Stop annotations target being rewritten from items

[JackLewis-digirati]

What does this change?

Resolves #694

This PR modifies how target is rewritten, so it rewrites a named query canvas before adding it, instead of working across all canvases after they've been added to the manifest. This means annotations set by the customer are left alone

[JackLewis-digirati]

How this fixes #694

The problem: after copying the named-query (NQ) canvas's annotation pages onto the canvas/manifest, AddRetargetedAnnotations looped over every annotation page on the resource. It retargeted any inline annotation that pointed at the NQ canvas. On a mixed manifest that includes pages the user supplied via items (or directly on the manifest), so a user annotation that deliberately targets the NQ canvas had its target silently rewritten.

The fix: the loop now runs over the NQ canvas's pages rather than the resource's pages. Pages already on the resource are never modified. For each NQ page:

• A page with the same id is already on the resource: it's skipped, and the existing page is left exactly as it is.
• Otherwise: the NQ page is cloned, retargeted and added. NQ pages are shared between every canvas the asset is painted on, so they're always cloned before modification.

The per-page retargeting (canvas target / #fragment / SpecificResource.Source, or the whole manifest for manifest-level adjuncts) is unchanged. It has moved into its own RetargetAnnotations method.

Behaviour changes:

• Pages already stored on the resource are no longer retargeted. This reverses Set the target to correctly point towards the canvas or the manifest #689's retargeting of stored pages, so if a user sets a target it's kept regardless.
• Newly added NQ pages are always cloned, not only when they contain an annotation that needs retargeting.

Tests:

• New: ProcessCanvasPaintings_DoesNotRetargetExistingInlineAnnotation_NotFromNamedQueryCanvas (canvas level) and MergeManifest_DoesNotRetargetExistingInlineAnnotation_NotFromStubCanvas (manifest level). In both, a user page targets the NQ/stub canvas: the user page keeps its target and the NQ page is still retargeted.
• Flipped from Set the target to correctly point towards the canvas or the manifest #689: ProcessCanvasPaintings_DoesNotRetargetExistingInlineAnnotationAdjunct_TargetingNamedQueryCanvas and MergeManifest_DoesNotRetargetExistingInlineAnnotation_TargetingStubCanvas. An existing page with the same id as an NQ page keeps its original target.