v3.18.1: the raw envelope action takes the source override too

The events without a semantic builder (MT call, IMS registration, slices, ...)
are network-sourced, but the raw `envelope` action had no `src` override - the
only way to express them was the semantic set.  `envelope` now accepts
`src` (82 terminal / 83 network) exactly like the `event` action; the step
editor exposes it and the authoring guide gets the row.

Also: the guide marks `por` as SCP80-only (the normaliser refuses it
elsewhere) and the semantic-events section names the shared src override.

Tests: 813 frontend + 717 Python; sw cache simple-v382.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "pysim-simple-server"
-version = "3.18.0"
+version = "3.18.1"
 description = "HTTP REST server wrapping pysim for the SIMple PWA"
 requires-python = ">=3.8"
 # pysim is a git-only dependency installed explicitly by setup.bat/setup.sh.