Fold the files derived from a document beneath it in the viewer

[SimonCropp]

A snapshot library that verifies a document often verifies what it computed from the document as well: a png and the text of each page, a csv per sheet. Each is a pending file of its own, so a reviewer accepts the document and then every file derived from it, even though the viewer's row for the document has already shown its pages and its text.

This lets a caller say that a pending file was derived from a pending source, and has the viewer fold derived files beneath the document they came from.

API

// The document first: what is derived from it names it
await DiffRunner.LaunchAsync(documentTempFile, documentTargetFile);

// A file computed from the document, named by the document's temp file
await DiffRunner.LaunchDerivedAsync(pageTempFile, pageTargetFile, documentTempFile, null);

// A file the document no longer produces
await DiffRunner.AddDerivedDeleteAsync(stalePageFile, documentTempFile);

Also LaunchDerived, LaunchDerivedForText, LaunchDerivedForTextAsync and AddDerivedDelete. Distinct names with no optional parameters, so no shipped signature changes (BinaryCompatibilityTests pins the old and the new).

Behavior

• When the tool resolved for the source is DiffEngineViewer and that copy draws documents, a derived file opens no tool, costs nothing against MaxInstancesToLaunch, and is tracked with its source.
• In every other case (the source is in Word or Beyond Compare, or in no tool) each file is launched exactly as before. Other diff tools are unaffected.
• In the viewer the document's row reads + Sample.Test (pdf) (5). Accept move +5 and Discard +5 take the document and everything beneath it, the document last. Expand in the row's menu, or a click on the row once it is selected, gives each derived file a row of its own.
• A page the document no longer has is a pending delete beneath it, carried out when the document is accepted.

Wire

Additive on all three, so an older tray, viewer or library yields ordinary rows and never a lost file:

• viewer port request: a source: line on move, diff and delete
• viewer port listing: a derived: key|source key line of its own, as held: is
• tray port: an optional trailing "Source" on the Move and Delete payloads, which are byte for byte what they were without it

The tray stores and relays the marker; its menu is unchanged. The fold and the combined accept are the viewer's.

Notes

• PiperTest.DeleteJson snapshotted a Move payload, so the "Add pending delete" sample in docs/tray.md showed a Move. It now shows a Delete.
• DocumentWatch.Reread dropped Written when it re-read a held delete. It is carried now.
• docs/diff-tool.md and diffTools.include.md pick up the 20.7.0-beta.1 version string. That is the test suite regenerating them, unrelated to this change.
• The version is not bumped here.
• Not run: the fold has been checked through the model, the ASCII screen and both wires, not in a window. Neither native head was run with one, and there is no pixel capture of a folded document. todo.md lists what is open.

The consumer is Verify 34, which reports the targets a converter derived from a document this way.