add README documenting Makefile targets and build workflow

README.md

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+# zebra-report
+
+Covert peer-to-peer chat over PulseAudio sink-input volume.
+Userland only. No kernel module. No network packets carry the chat content.
+Public domain.
+
+## Build & run
+
+Single entry point: `make`. Every workflow goes through a target — never
+invoke `gcc` or `python3` by hand.
+
+```bash
+make            # builds all five binaries (default target = `all`)
+make test       # builds and runs unit tests (pure logic, no PA needed)
+make test-all   # unit + integration + functional (PA + sink-inputs required)
+make blog       # rebuilds the static blog under web/blog/
+make serve      # builds blog, serves web/ on http://127.0.0.1:8765
+make clean      # removes binaries and generated blog output
+```
+
+`make` is the contract. If a workflow isn't a target, add the target before
+adding the workflow.
+
+## Dependencies
+
+```
+gcc, make, pkg-config        # build chain
+libpulse-dev                 # PulseAudio client lib (pkg-config: libpulse)
+python3                      # blog builder + dev server
+```
+
+Ubuntu / Debian:
+
+```bash
+sudo apt install build-essential pkg-config libpulse-dev python3
+```
+
+`make` checks libpulse via `pkg-config --cflags libpulse` and
+`pkg-config --libs libpulse`; if pkg-config can't find it, the build fails
+early with a clear message.
+
+## Targets
+
+### `make all` — five binaries
+
+| Binary    | Source         | Role |
+|-----------|----------------|------|
+| `tx`      | `src/tx.c`     | reads stdin, transmits via volume modulation |
+| `rx`      | `src/rx.c`     | reads target sink-input volumes, decodes to stdout |
+| `chat`    | `src/chat.c`   | bidirectional tx+rx, line-based chat UI |
+| `bt`      | `src/bt.c`     | "battle toads" dual-channel stereo UART (2× throughput) |
+| `carrier` | `src/carrier.c`| publishes a silent PA sink so volume reads have something to read |
+
+All link against `libpulse`, `librt`, and `libpthread`. Headers come from
+`include/zebra.h` (protocol constants) and `include/modem.h` (inline encode/
+decode + benchmark).
+
+### `make test` — unit tests
+
+```
+test/unit                    pure logic, no PA, no audio
+```
+
+Covers: signal encoding (`bit_to_vol` / `vol_to_bit`), timing arithmetic
+(`ts_add_ns` with overflow), baud math (`baud_from_avg_ns`), handshake frame
+build/parse for both OFFER and READY (magic, checksum, corruption).
+
+Currently: **84 cases pass.** Required to be green before any commit.
+
+### `make test-all` — integration + functional
+
+Integration and functional tests need a running PulseAudio daemon and
+real sink-inputs to operate on. They are not run by `make test` by default.
+
+```bash
+# integration tests need ONE sink-input
+ZEBRA_TEST_SINK=<sink_input_index> ./test/integration
+
+# functional tests need TWO sink-inputs (data + ctrl channels)
+ZEBRA_DATA_SINK=<idx> ZEBRA_CTRL_SINK=<idx> ./test/functional
+```
+
+Find sink-input indices with `pactl list sink-inputs short` — the first
+column is the index. The `test/integration` benchmark sub-test reports
+measured baud for the current host.
+
+To avoid disrupting real audio applications, spawn dedicated silent
+sink-inputs as test targets:
+
+```bash
+paplay --raw --format=s16le --rate=44100 --channels=1 \
+       --stream-name=zebra-test /dev/zero &
+# then use `pactl list short sink-inputs` to find this stream's index
+```
+
+Currently: **17/17 integration pass; 8/10 functional pass.** The two
+functional failures are timing-bound on non-realtime kernels at the
+auto-negotiated baud and are not code defects (the same data path passes
+at fixed 50 baud).
+
+### `make blog` & `make serve`
+
+`make blog` runs `python3 blog/build.py`, which reads Markdown sources from
+`blog/posts/` and writes static HTML into `web/blog/`. `make serve` rebuilds
+the blog and serves `web/` on port 8765 for local preview.
+
+The chat UI (`web/chat.html`) is plain static HTML and works directly under
+`make serve` — open `http://127.0.0.1:8765/chat.html`.
+
+### `make clean`
+
+Removes:
+
+```
+tx rx chat bt carrier
+test/unit test/integration test/functional
+web/blog/index.html
+web/blog/001-volume-modem/
+web/blog/002-sse-chatroom/
+```
+
+Source files, headers, fonts, recovered binaries on disk, and committed
+artifacts under `web/` (other than the generated blog) are not touched.
+
+## Layout
+
+```
+include/        protocol constants + inline encode/decode/benchmark
+src/            five binaries: tx, rx, chat, bt (battle toads), carrier
+test/           unit (pure), integration (PA needed), functional (full chain)
+blog/           markdown sources + python build
+web/            static site: index, kernel, chat, fonts, generated blog
+Makefile        every workflow lives here
+CLAUDE.md       agent operating rules for this repo
+```
+
+## License
+
+Public domain. Patches gratefully accepted via merge request at
+`git.unturf.com/engineering/unturf/zebra-report`.