Map import facts (source file, source URL) to destination fields

[deanleigh]

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Implements #109, and generalises it so the web/URL case falls out of the same mechanism.

The problem

Every mapping today is source element -> destination field: a rule matches content in the document, text comes out, it is written (coerced for number/date).

brochurePdf is not that. Nothing in the PDF content is captured. The value is a fact about the import itself: which file the editor picked in the Create from Source picker. The URL case for web imports is the same shape, with sourceUrl instead of mediaUnique.

So this is a second kind of mapping source: import facts, alongside captured content.

Current blockers

1. DestinationStructureService maps Umbraco.MediaPicker3 -> mediaPicker (line ~463) but TextMappableTypes (line ~36) does not contain it, so the field is classified and then dropped. brochurePdf never reaches the Destination tab.
2. map.json has no way to express a source that is not an element ID.
3. The apply pass never writes it.

Not a blocker

mediaUnique (the picked media key) is already on UmbUpDocModalValue and is already destructured in both bridge files (up-doc-action.ts:122, up-doc-collection-action.element.ts:155). It is currently used only as a guard and then discarded. The value is already in scope where it needs to be written.

Decisions taken

• Two sets, not one widened set. TextMappableTypes keeps its name and meaning (filled by rules). A new set covers types fillable from import facts. A type may appear in both (a URL goes into a plain text field), so eligibility is "in either set", and the mapping records which kind it is.
• DestinationField must carry the distinction so the Destination tab picker offers "the source file" rather than the list of extracted elements.
• Adding mediaPicker cannot pollute the Extracted tab - that view is built from source rules, and destination.json has no influence on it.

MediaPicker3 value shape

Confirmed against real content in the mirror (TailoredTravel.Web/uSync/v17/Content/ttm5063.config):

[
  {
    "key": "a2ad1a8b-b304-4cf0-a145-20938c8f9890",
    "mediaKey": "d155485a-76a9-4d33-935c-146469ed9087",
    "mediaTypeAlias": "umbracoMediaArticle",
    "crops": [],
    "focalPoint": null
  }
]

key is a fresh GUID per picker entry; mediaKey is the media item. mediaTypeAlias varies per media item and must be resolved, not hardcoded.

Scope

• Add the import-fact type set; include mediaPicker
• Surface the distinction on DestinationField
• Express an import-fact mapping in map.json
• Destination tab: offer import facts as a source when the field is import-fillable
• Apply pass writes MediaPicker3 shape (both bridge files, since Extract duplicated apply logic from the two bridge files #41 is outstanding)
• Verify end to end against a real tour PDF

URL/web targets use the same mechanism and follow once the PDF case is proven.

Notes

• Both bridge files must stay in sync (Extract duplicated apply logic from the two bridge files #41).
• Rebuild dist/updoc.js after TS changes.
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