Organize overlay tests by behavior

packages/rsnap-overlay/src/overlay/tests/README.md

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+# Overlay Test Layout
+
+`tests.rs` owns shared fixtures, helper functions, and module declarations only.
+New overlay behavior tests should live in the narrowest matching behavior module:
+
+- `annotation_runtime.rs`: frozen annotation editing, text input, brush modeling, undo/redo.
+- `export_actions.rs`: PNG/OCR/export authority and host-effect requests.
+- `live_runtime.rs`: live-mode pointer, loupe, hotkey, and native input routing.
+- `rendering_behaviors/`: visual/layout contracts split by rendering domain.
+- `scroll_capture_runtime.rs`, `scroll_input_runtime.rs`, `stream_refresh_runtime.rs`: scroll capture runtime state, input freshness, and stream refresh policy.
+- `self_capture_runtime.rs`: self-capture filtering, window matte capture, and deferred worker refresh.
+- `toolbar_runtime.rs`: toolbar window visibility, pointer state, and drag eligibility.
+- `worker_*_runtime.rs`: worker scheduling and observation contracts.
+
+Prefer table-driven coverage when cases share the same entry point, branch shape,
+and externally observable result. Keep separate tests when similar fixtures protect
+different user-visible contracts, state transitions, filesystem/host effects, or
+failure classifications.