fix(net): backport single-group FETCH on draft-20 and later

doc/concept/standard.md

@@ -50,12 +50,14 @@ to model priority 127, where higher values are served first. A track that
 never sets a priority is 127 as well, so it goes out as 128 on IETF and 127
 on moq-lite.
 
-On drafts 14–19, the Rust publisher answers a standalone `FETCH` within one
+On drafts 14–22, the Rust publisher answers a standalone `FETCH` within one
 group from the cache. A relay fetches a missing group upstream with a `FETCH`
 of that one whole group, and an upstream refusal is the refusal the fetcher
-sees. A range touching several groups is refused with `NOT_SUPPORTED`, as is
-any `FETCH` on draft-20 and later, which moved the range into
-`LOCATION_FILTER`. A standalone `FETCH`
+sees. A range touching several groups is refused with `NOT_SUPPORTED`, as is a
+draft-20+ `LOCATION_FILTER` that ends at Largest Object instead of an absolute
+End Location. On draft-20 and later, `FETCH_OK` reports the inclusive requested
+end, capped at Largest Object when known. An empty range gets an empty fetch
+stream; a start past Largest Object is refused with `INVALID_RANGE`. A standalone `FETCH`
 carries no timestamps, since no `SUBSCRIBE_OK` declared a timescale for it.
 
 On drafts 14–19, the Rust publisher also serves relative and absolute joining