feat: Add streaming pipe support for async loading in.

docs/DEVPLAN.md

@@ -170,7 +170,7 @@ through hooks and structured I/O. A handler hook can
 receive hover events and emit commands to modify menu
 state.
 
-## Phase 4: Multi-Select
+## Phase 4: Multi-Select ✓
 
 Selection with undo/redo. No marks or registers: filtering
 already handles navigation, and selections surviving filter
@@ -275,21 +275,30 @@ pikl instance over the socket, push items, read state, and
 subscribe to events. A named session remembers filter
 history across invocations.
 
-## Phase 7: Streaming & Watched Sources
+## Phase 7: Streaming Stdin ✓
 
-Live, dynamic menus.
+Auto-detected streaming when stdin is a pipe (not action-fd,
+not CSV/TSV). The menu opens immediately with zero items and
+streams them in as they arrive.
+
+`--watch` was dropped from this phase. External tools like
+`inotifywait -m ~/walls | pikl` compose naturally with
+streaming stdin. No built-in file watcher needed.
 
 **Deliverables:**
-- Async/streaming stdin (items arrive over time, list
-  updates progressively)
-- Streaming output (on-hover events emitted to stdout)
-- `--watch path` for file/directory watching
-- `--watch-extensions` filter
-- notify crate integration (inotify on Linux, FSEvents
-  on macOS)
+- Auto-detected streaming stdin (items arrive over time,
+  list updates progressively)
+- `StreamingDone` action signals end of input
+- `streaming` flag on ViewState for frontend indicators
+- TUI shows `...` suffix on the count while streaming
+- Background reader with batched `AddItems` (up to 100
+  items per batch, flushes immediately for slow sources)
+- `streaming-done` action-fd command for test scripts
+- No new CLI flags: streaming is auto-detected
 
-**Done when:** `find / -name '*.log' | pikl` populates
-progressively. A watched directory updates the list live.
+**Done when:** `seq 1 10000 | pikl` opens immediately and
+items stream in. Slow sources show items appearing one at
+a time with a `...` indicator.
 
 ## Phase 8: GUI Frontend (Wayland + X11)