feat: documents and session tokens

fixtures/src/README.md

@@ -1,6 +1,7 @@
 # Fixture sources
 
-Each source describes a document as first read, errors included, in a shorthand that `tools/build.py` expands; the build mints IDs, applies the listed corrections, lays out synthetic geometry, signs the listed receipts and writes the package.
+Each source describes a document as a converter first read it, mistakes included, in a shorthand that `tools/build.py` expands.
+The build mints IDs, applies the listed corrections, lays out synthetic geometry, signs the listed receipts and writes the package.
 The shorthand is tooling, not a format.
 
 | Shorthand | Becomes |