feat!: ship receiver-only .deb; deprecate C sender (replaced by srtla-send-rs)

[andrescera]

Affected repo & language: srtla — CMake/C++ build + CI (GitHub Actions), docs

What

The srtla package becomes receiver-only: it now installs srtla_rec only. The C srtla_send is dropped from the install/package payload — the CMake install() rule no longer lists it, and the FPM .deb step no longer ships the srtla-send.service systemd unit. The C sender source and its full GTest suite stay in-tree and still build and run; only packaging changes. Docs (AGENTS.md, README.md) mark the C sender deprecated and point to srtla-send-rs for the sender binary.

Why

Per ADR-003, the device-side sender has moved to the Rust fork srtla-send-rs (v1.0.0 released), which is CLI- and telemetry-compatible. Shipping the C srtla_send alongside it would double-provide /usr/bin/srtla_send and conflict with the fork's package. Making srtla receiver-only lets the two packages coexist cleanly on the device image: srtla provides srtla_rec, srtla-send-rs provides srtla_send. The C sender is retained in the tree as the protocol reference and to keep the compat harness' C-sender pairs runnable (Rule E — no tests deleted).

How to verify

• Payload (receiver-only): build the .deb the way publish-release.yml does and list it:

  cmake -B build-pkg -DCMAKE_BUILD_TYPE=Release -DSRTLA_BUILD_TESTS=OFF -DCMAKE_INSTALL_PREFIX=/usr
  cmake --build build-pkg -j"$(nproc)"
  DESTDIR=$PWD/install cmake --install build-pkg
  fpm -s dir -t deb -n srtla -v 0.0.0 -a amd64 -p srtla.deb install/usr/=/usr/
  dpkg -c srtla.deb            # → ./usr/bin/srtla_rec present; NO srtla_send; no srtla-send.service

• Tests stay green / sender still builds: a normal build still produces both binaries and the suite is intact:

  cmake -B build -DCMAKE_BUILD_TYPE=Release && cmake --build build -j"$(nproc)"
  ls build/srtla_send build/srtla_rec      # both present (target retained)
  (cd build && ctest --output-on-failure)  # 178/178 across the 19 test_*.cpp suites

Risks

• Device image must also install srtla-send-rs to get a sender — srtla alone no longer provides one. This is the planned cutover (ADR-003); the fork's .deb already Conflicts/Replaces: srtla (<< <cutover-version>), and that bound must be set to this receiver-only srtla release's version so the packages coexist.
• No receiver code, no protocol/wire, and no TS bindings change. Rollback is a one-line revert of the install() rule and the FPM payload line.