docs: LEARNING.md + fuzz/ placeholder + README dev-loop (spec §7, §6.3)

LEARNING.md indexes ten concept-to-file pointers (the spec floor was
five) — the newtype pattern, exhaustive enum match, sans-IO, trait
extension seams, thiserror + hot-path match-and-continue, Arc<Mutex>
vs Arc<RwLock>, DashMap, str0m's single-mutation invariant, graceful
shutdown, include_str!. fuzz/README.md pre-paves the layout (no
hostile-bytes surface in slice 1; harnesses land at step 5 per the
out-of-scope table). README's new dev-loop section documents the
libopus FFI prerequisite and the manual e2e steps.

README.md

@@ -8,6 +8,48 @@ or its architecture.
 > technical builder uses to stand up a contact center* — and re-aims it at a category AI is
 > actively disrupting, instead of a PBX category UCaaS already ate.
 
+## Slice 1 dev loop (WebRTC media loopback)
+
+> Build prerequisite: install libopus (the `opus` crate links it via FFI):
+> ```bash
+> sudo apt-get install -y libopus-dev   # Debian/Ubuntu
+> # Fedora: sudo dnf install -y opus-devel
+> # macOS:  brew install opus
+> ```
+> This is the one system dependency in slice 1. Opus is FFI per PORT_PLAN
+> §7's "🦀 Core (FFI)" disposition — the codec surface Rust doesn't need
+> to re-implement.
+
+Run the server:
+
+```bash
+cargo run
+# listening on http://0.0.0.0:8080
+```
+
+Open a browser to `http://localhost:8080/`, click "Start call", grant
+microphone permission. Speak — you should hear yourself back within
+~200 ms (no perceptible delay). Click "Hang up" to tear down; server
+logs `Closing → Closed`.
+
+Verbose tracing:
+
+```bash
+RUST_LOG=rutster=debug cargo run
+```
+
+### Slice 1 "done" checklist (spec §6.5)
+
+On a clean checkout:
+1. `cargo test --all` passes.
+2. `cargo fmt --check` passes.
+3. `cargo clippy -- -D warnings` passes.
+4. `cargo deny check` passes.
+5. `cargo run` + browser manual e2e: speak → hear echo within ~200 ms.
+6. Hang-up button triggers `Closing → Closed` in server logs.
+7. Every stub crate compiles; its doc-comment names its scheduled step.
+8. `LEARNING.md` indexes at least 5 "to learn X, read Y" pointers.
+
 ## Why it exists
 
 Asterisk won because contact centers were **built on it** (Vicidial, GOautodial, a thousand