fix: refuse a base container left concrete beside its own children

[mysma-9403]

abstract defaults to false in XTCE, so a base SequenceContainer that nobody marked —
the ordinary CCSDS primary-header shape, which is most definitions — is emitted as a concrete
container. Its generated matches is true for any buffer long enough to hold it, and the
module-level decode counts how many containers match:

if FlightPacket::matches(data) { matched += 1; … }   // body is `true`
if StatusReport::matches(data) { matched += 1; … }
if matched > 1 { return Err(DecodeError::Ambiguous) }

So every packet matched the base and its own child, and decode answered Ambiguous for
all of them. A generated decoder that rejects every packet, from a definition that generated
without a word of complaint — and the place you would find out is the spacecraft.

Refused at generation time now, naming the container and giving both ways out:

$ xtce-flight generate concrete_base.xml -o out.rs
error: cannot compile <SequenceContainer> in FlightPacket: this container has no restriction
criteria, so it matches every packet long enough to hold it; beside the other containers
generated here that makes every packet ambiguous. Mark it abstract="true" if other containers
inherit from it, or give it criteria that select it

Narrow on purpose: only when there is more than one container. A lone concrete container with
no criteria is the whole definition and decodes fine, so it still generates.

Reproduced against the bundled definition by removing one attribute from it; flight_shapes.xml
itself still generates its two structs. 43 tests, fmt and clippy clean.