Add alignment state indicator and pluggable data sources

- AlignmentThresholds dataclass + VisionEngine.__init__ for injection
- alignment_state computed per snapshot (ready/aligning/fault)
- MonitorPlot circle/vector/legend driven by state color
- Status dot + label moved into POSITION VISUALIZATION panel header
- CsvSessionSource + DemoSource pluggable DataSource layer
- Synthetic DATA/ready/ and DATA/aligning/ sessions for visual testing
- Fix CAM 3 label placement (below circle, not above)

src/wafer_edge/DataSources.py

@@ -0,0 +1,103 @@
+"""Pluggable data sources for the wafer monitor.
+
+A DataSource produces a MonitorSnapshot on demand. Today: CSV session + synthetic
+demo. Future: live MQTT from NanoBerry. App.py only talks to this interface.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Protocol
+
+from .Calibration import BaselineCalibration
+from .CsvSource import find_session_files, load_camera_records_from_csv
+from .VisionEngine import MonitorSnapshot, VisionEngine
+
+
+class DataSource(Protocol):
+    def latest_snapshot(self, baseline: BaselineCalibration) -> MonitorSnapshot: ...
+
+    def description(self) -> str: ...
+
+
+# ---------------------------------------------------------------------------
+# P2.2 — CsvSessionSource for replaying a selected session.
+#
+# Resolves sibling files once from the seed path using P1.1, then re-reads
+# those files on every latest_snapshot() call. If discovery returns no files,
+# raises FileNotFoundError so a bad operator selection is visible immediately.
+# ---------------------------------------------------------------------------
+class CsvSessionSource:
+    """Reads a captured session from a directory of CSV files.
+
+    The operator selects the session folder (e.g. ``sessions/0540/``); this
+    source discovers all cameras inside it automatically.  Re-reads files on
+    every ``latest_snapshot()`` call so "Reload Current Session" always reflects
+    the latest on-disk state without cache invalidation.
+    """
+
+    def __init__(self, folder: Path, engine: VisionEngine) -> None:
+        self.folder = Path(folder)
+        self.engine = engine
+        # Discover once at construction so missing-folder errors are immediate.
+        paths = find_session_files(self.folder)
+        if not paths:
+            raise FileNotFoundError(f"No NanoBerry CSV files found in {self.folder}")
+        self._cam_count = len(paths)
+
+    def latest_snapshot(self, baseline: BaselineCalibration) -> MonitorSnapshot:
+        # Re-resolve on every call — supports "Reload Current Session".
+        paths = find_session_files(self.folder)
+        if not paths:
+            raise FileNotFoundError(f"No NanoBerry CSV files found in {self.folder}")
+        self._cam_count = len(paths)
+        records = load_camera_records_from_csv(paths, baseline)
+        return self.engine.snapshot_from_records(
+            records,
+            baseline,
+            source_label=self.description(),
+        )
+
+    def description(self) -> str:
+        return f"Session: {self.folder.name} ({self._cam_count} cams)"
+
+
+# ---------------------------------------------------------------------------
+# P2.3 — DemoSource as the default safe source.
+#
+# First launch should not depend on MASTER_SD being present or on the working
+# directory. Wrapping VisionEngine.demo_snapshot() behind the same protocol
+# means demo mode, CSV replay, and future MQTT all flow through P3.1 with no
+# special-case read logic. The description is stable because P3.3 displays it
+# directly in the status bar.
+# ---------------------------------------------------------------------------
+class DemoSource:
+    """Synthetic perfect-world data — always works, used as fallback."""
+
+    def __init__(self, engine: VisionEngine) -> None:
+        self.engine = engine
+
+    def latest_snapshot(self, baseline: BaselineCalibration) -> MonitorSnapshot:
+        return self.engine.demo_snapshot(baseline)
+
+    def description(self) -> str:
+        return "Demo: Simulated Data"
+
+
+# ---------------------------------------------------------------------------
+# TODO P6.1: Post-scope hook for MqttSource implementing DataSource.
+#
+# THINKING:
+#   Live MQTT should enter the app as another DataSource, not by changing
+#   VisionEngine or App.read_wafer(). The source can own broker connection,
+#   background subscription, and latest-frame buffers, then convert payloads to
+#   CameraEdgeRecord before calling snapshot_from_records(). Keeping this as a
+#   P6 task protects the beta session-loader milestone from broker/threading
+#   risk while documenting the intended extension point.
+#   Cross-reference: P6.2 adds the future App.py action that constructs it.
+#
+# Future signature:
+#   class MqttSource:
+#       def __init__(self, broker: str, engine: VisionEngine) -> None: ...
+#       def latest_snapshot(self, baseline: BaselineCalibration) -> MonitorSnapshot: ...
+# ---------------------------------------------------------------------------