Add questions, parts and solutions by source range

CHANGELOG.md

@@ -7,4 +7,5 @@
 - beartype is now `^0.22`. At 0.20.0 and below its import hook leaves `cli` a plain function rather than a group, so the new command line either fails to import or runs `convert` whatever the arguments; 0.20.1 is the first version that works.
 - `in2lambda source add FILE` freezes a document: it converts .docx and .tex to markdown beside the file, and writes a `draft.json` holding the markdown's hash and every block in it with the lines it spans, so that another tool can quote the source by line range. `in2lambda source show` prints that markdown numbered with the block ids. Freezing a file that has changed since is refused unless `--start-over` says to discard the draft, and so is showing one, since its block ids would name lines they are not the ids of. Both need pandoc and the `convert` extra, as `convert` does.
 - A draft now holds a `log` of every command that changed it and a `fields` map of what those commands wrote, each field recording which layer wrote it (1 a spec, 2 a predicate, 3 a line range, 4 a literal), the source ranges it was copied from, whether it has been edited and by whom. `in2lambda draft mark ignore BLOCK` is the first such command, and `in2lambda draft replay` rebuilds the draft from the frozen markdown and the log, refusing unless what it builds is the `draft.json` that is there, byte for byte. A `draft.json` written before this has no `log` in it and is refused as one nothing here wrote; `in2lambda source add --start-over` freezes the document again.
+- A draft is filled in by `in2lambda draft question add`, `in2lambda draft part add QUESTION` and `in2lambda draft question solution QUESTION`. Each takes `--text` to copy the wording out of the frozen source, as a block id such as `b3` or as lines such as `s10:14`, or `--literal TEXT` where the source does not say it in a form the field can take, which records the field as edited and written by layer 4 rather than 3. Question and part numbers are worked out from the fields already written rather than given, so a replay arrives at the same ids. `in2lambda draft split block BLOCK AT` cuts a block the parser made one of two things into `b3a` and `b3b`, so that each half can be quoted on its own. A command writing a field that is already written, or quoting lines another field was taken from, is refused naming both fields.
 - The Python API is unchanged: `in2lambda.main.runner` and everything under `in2lambda.api` take the same arguments and return the same objects.