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.

docs/api.md

@@ -666,7 +666,8 @@ server resolves it from the preset store, so a run always uses the stored
 keysets, TARs and counters.  The inline `preset` form stays for external
 callers.
 
-**Action steps** (`type: "action"`): `kind` is `envelope` (`event`, `data`),
+**Action steps** (`type: "action"`): `kind` is `envelope` (`event`, `data`,
+optional `src` - the device-identities source override),
 `event` (`event` name or hex + `fields`, optional `src` - the semantic event
 download for 0x03/0x0B/0x12/0x1D), `menu-select` (`item_id` 1-255, or `text` +
 `mode` `exact`/`contains` with an