feat: alignment pairs a misread sentence with its other reading; a suspect names its witness

spec/05 §5.6 gains a substitution step: an unpaired sentence beside one at
least 0.8 alike (2L/(|a|+|b|) over the lossy projections' code points) is
paired with it, so a misread is one number, equation or word suspect rather
than a dropped and an added one. §5.7 and suspect.schema.json gain an
optional `witness`, checked by doc.suspect.witness. alignment.jsonl gains a
`substitutions` case; the first case is unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

vectors/README.md

@@ -138,7 +138,7 @@ A candidate format changes only with a recorded reason and a new or changed vect
 
 A passing run shows one thing: for these inputs, your implementation computes the same values as the reference implementation in `src/paperspec/`.
 That code computed the expected outputs, and `pixi run build` regenerates them ([tools/](../tools/README.md)).
-The vectors hold 311 cases in all, some of them small: `hashes.jsonl` has one document, `alignment.jsonl` one pair of readings, `offsets.jsonl` two cases.
+The vectors hold 312 cases in all, some of them small: `hashes.jsonl` has one document, `alignment.jsonl` two pairs of readings, `offsets.jsonl` two cases.
 
 The vectors do not show the following.