feat: host-agnostic StrokeHost (CoordinateSpaceHost, plan principle #2)

Ties the engine together: a StrokeHost is anything ink attaches to — a PDF page,
an infinite-board region, or a (P5) CAS overlay — with a stable hostId (cache +
persistence key), a contentSize (normalized↔px mapping), and a revision-tracked
StrokeStore. strokesIn(viewport) broad-phase-culls via stroke_bounds for the
board. The viewport mounts one AnnotationLayer per host; nothing in the engine
knows page vs board (the one host-agnostic ink engine).

Makes StrokeStore (P0) + stroke_bounds load-bearing together. Pure; unit-tested.

flutter analyze lib/editor clean; 208/208 tests (+4).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/editor/engine/stroke_host.dart

@@ -0,0 +1,52 @@
+// lib/editor/engine/stroke_host.dart
+//
+// A CoordinateSpaceHost (plan principle #2: ONE host-agnostic ink engine). A
+// host is anything ink attaches to — a PDF page, an infinite board region, or a
+// (P5) CAS overlay — identified by [hostId], with a [contentSize] that defines
+// the normalized↔pixel mapping, and a committed [StrokeStore]. The viewport
+// mounts one AnnotationLayer per host; nothing in the engine knows whether it's
+// a page or a board.
+//
+// Pure (no widgets/pdfrx); ties together StrokeStore (P0) + stroke_bounds
+// broad-phase culling. Unit-tested.
+
+import 'dart:ui' show Rect, Size;
+
+import 'stroke_bounds.dart';
+import 'stroke_model.dart';
+import 'stroke_store.dart';
+
+/// One ink host: identity + content geometry + its committed strokes.
+class StrokeHost {
+  StrokeHost({
+    required this.hostId,
+    required this.contentSize,
+    StrokeStore? store,
+  }) : store = store ?? StrokeStore();
+
+  /// Stable id (e.g. `"doc:<id>:page:<n>"` or a board-region id) used as the
+  /// ink Picture cache key prefix + the persistence host id.
+  final String hostId;
+
+  /// Content size in logical px at scale 1; normalized [0,1] coords map onto it.
+  final Size contentSize;
+
+  /// Committed strokes for this host (revision-tracked).
+  final StrokeStore store;
+
+  /// Revision of the committed strokes (O(1) repaint gate passthrough).
+  int get revision => store.revision;
+
+  /// Committed strokes whose bounds overlap [viewportNormalized] — broad-phase
+  /// culling for the infinite board (skip off-screen strokes). For a bounded
+  /// PDF page the whole page is usually in view, so callers can skip this.
+  List<EditorStroke> strokesIn(Rect viewportNormalized) {
+    return [
+      for (final s in store.committed)
+        if (strokeIntersects(s, viewportNormalized)) s,
+    ];
+  }
+
+  @override
+  String toString() => 'StrokeHost($hostId, $contentSize, rev=$revision)';
+}