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>

schemas/suspect.schema.json

@@ -60,6 +60,9 @@
         }
       ]
     },
+    "witness": {
+      "$ref": "common.schema.json#/$defs/nodeId"
+    },
     "crop": {
       "$ref": "common.schema.json#/$defs/hash"
     },