docs(bf-3p8h): verify spawn_stream_json_reader signature and retry logic

notes/bf-3p8h.md

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+# Verification of spawn_stream_json_reader
+
+## Function Signatures
+
+### Primary public function (src/emitter.rs:98-100)
+```rust
+pub fn spawn_stream_json_reader(transcript_path: PathBuf, start_offset: u64) -> StreamJsonHandle
+```
+
+### Testable variant with writer injection (src/emitter.rs:103-116)
+```rust
+pub fn spawn_stream_json_reader_to(
+    transcript_path: PathBuf,
+    start_offset: u64,
+    writer: Box<dyn Write + Send + 'static>,
+) -> StreamJsonHandle
+```
+
+Both return a `StreamJsonHandle` containing:
+- `drain_tx: mpsc::SyncSender<()>` - channel to signal "drain then exit"
+- `join_handle: thread::JoinHandle<()>` - thread handle for joining/waiting
+
+## Retry Logic Verification (src/emitter.rs:127-149)
+
+**Confirmed: 50ms retries up to 5 seconds are correctly implemented.**
+
+The retry logic in `stream_json_reader_loop` works as follows:
+
+1. **5-second deadline**: `let deadline = std::time::Instant::now() + Duration::from_secs(5);`
+2. **Retry loop** with 50ms sleep: `thread::sleep(Duration::from_millis(50));`
+3. **On `File::open` failure**:
+   - First checks for drain signal via `drain_rx.try_recv()` → exit if received
+   - Then checks if 5-second deadline expired → exit if timed out
+   - Otherwise sleeps 50ms and retries
+4. **On success**: `break f` exits the retry loop with the opened `File`
+
+The logic correctly handles:
+- File appearing mid-retry (opens and continues)
+- Drain signal during retry (exits immediately)
+- Timeout after 5 seconds (exits, file never appeared)
+
+## mpsc Channel Drain Verification (src/emitter.rs:157-194)
+
+**Confirmed: The function correctly drains to the mpsc channel.**
+
+The channel usage:
+- **Creation**: `mpsc::sync_channel(1)` - bounded sync channel with capacity 1
+- **Normal mode**: Continuously reads lines and writes to writer
+- **Drain signal**: Sending `()` via `drain_tx` sets `draining = true`, causing the loop to finish the current line then exit
+- **Immediate exit**: Dropping `StreamJsonHandle` without sending causes `Disconnected` error → immediate return
+
+## Summary
+
+All acceptance criteria verified:
+- ✅ Function signature documented (takes `PathBuf` transcript path and `u64` start_offset, returns `StreamJsonHandle`)
+- ✅ 50ms/5s retry logic confirmed in reader loop
+- ✅ mpsc channel drain logic confirmed correct
+- ✅ No code changes required