Launch the viewer without the test host's handles, and settle by value

[SimonCropp]

This fixes the last two "Library and inline" items from todo.md, which #894 left open because each needed a design decision. That section of todo.md is now empty and removed.

dotnet test does not return until the launched viewer is closed

The viewer inherited the pipe dotnet test reads the test host's output from, and dotnet test reads it until every writer has closed it.

• Windows: the launch uses ShellExecute, which inherits no handles. A process started without it inherits every inheritable handle, whatever is redirected, which is why redirecting all three streams still hung. Measured with a stand-in host and a 15 s child: 16.2 s to end of output before, 1.2 s after.
• Linux and macOS: .NET opens every descriptor close-on-exec, so only the standard streams pass to a child. All three are redirected and this side's ends are closed.
• The patch: a ShellExecute launch cannot redirect stdin, so the patch goes in a temp file named by --payload. The viewer reads the file and deletes it. With no --payload, it still reads stdin, so the documented < the.inlinepatch form and an older DiffEngine keep working.
• Windows head: it is now a WinExe. The SDK only infers that for a -windows framework, so the apphost was a console executable, and ShellExecute would have given it a console window beside the viewer. It attaches to its parent's console only to print a usage error.

Tests: ViewerLauncherTests.OnWindowsTheViewerInheritsNothing, ElsewhereTheViewerHasStandardStreamsOfItsOwn, AnInlineLaunchNamesItsPayloadFile, and CommandLineTests.InlineWithPayload, APayloadFileIsReadAndDeleted, AMissingPayloadFileIsNoPayload. The stand-in host measurement is not a committed test: a real one would open a console window during the run.

Settle-by-member counts queue entries, not call sites

A settle can now carry the passing call's value, through a new optional value argument on DiffRunner.SettleInline and a value field on the wire. When the line names no entry, the member fallback then only takes an entry that value settles (InlinePatch.IsSettledBy): one anchored to it, or one waiting to become it. A value also picks one entry out of several in a member. InlineStaging.Clear takes the same value for the same fallback on disk.

Without a value, behaviour is unchanged, so nothing regresses until Verify passes one. An owner that predates the field skips it, as it skips any unknown field.

Tests: InlineQueueTests.SettleFromAPassingSiblingKeepsTheFailingSiblingsEntry, SettleByMemberTakesAnEntryAnchoredToTheValue, SettleByMemberTakesAnEntryWaitingForTheValue, SettleByMemberWithAValueChoosesAmongSeveral, ASettleValueRoundTrips, and InlineStagingTests.ClearFromAPassingSiblingKeepsTheFailingSiblingsStagedTrio, ClearByMemberTakesACallSiteTheValueSettles.

Verify needs a follow-up: pass the expected value to SettleInline (after SnapshotValue for F#) and to InlineStaging.Clear.

The full solution passes locally in Release: 2,154 passed, 22 skipped.