The periodic activity announcement was gated only by RX_ONLY, which
defaults to off, so a stock ingestor deployed to feed a map starts
broadcasting on its community's default channel without the operator
having chosen to. The only opt-out, RX_ONLY=1, also silences the
MeshCore telemetry polls, so "ingest but stay quiet" was not expressible.
Add an ANNOUNCE boolean (default 0) as an opt-in gate on top of RX_ONLY,
which keeps its meaning as the global TX kill switch and overrides
ANNOUNCE=1. Amend SPEC MA7 (which had explicitly ruled out a separate
enable env) and MA-A7 with the reasoning, document both flags in the
README env table — the RX_ONLY row did not mention the announcement at
all — and add gate unit tests for all four combinations plus the
missing-attribute fail-closed path.
