Add a solutions document to a draft

[peterbjohnson]

Many sheets in the corpus come as two files: the questions and, separately, the solutions (the EEE_Mathematics folder is 23 such pairs; in2lambda convert already takes -a for this). A draft freezes one source, so the solutions cannot be quoted by range into it.

Let a draft hold more than one frozen source: in2lambda source add solutions.tex --into DRAFTDIR (or a second positional) freezes it beside the first as source 2, with its own hash and block list, block ids and line ranges qualified by source (the design spec's worked example writes ranges as s1:14, which reads as source 1 line 14; settle a syntax that keeps today's s99:119 meaning source 1 and adds s2:14 or similar for the second, and say so in the docs). source show prints both, numbered. Spec run's after Header text=Solutions selectors and the layouts pair solutions across sources the way convert -a does. Done when a fixture pair of question and solution files produces a draft whose part solutions are layer 1 fields quoted from the second source, and replay reproduces it.

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Workbench ticket t40.