Adopt Backdrop as the adaptive Haze input

HazeInput.Sources remains the exact source-capture input. HazeInput.Backdrop is the normal input for a built-in Blur or Glass surface whose intent is to consume the pixels already drawn behind it in the current window:

Modifier.hazeBlur(
  input = HazeInput.Backdrop(hazeState),
)

The native path samples the combined earlier pixels in the same window surface. It cannot select individual Haze sources, include later drawing, or cross a dialog, popup, or other window boundary. When the native path is unavailable, Haze uses Backdrop's fallback source input. Its selection and retention policies apply only to that source-capture path; they do not filter native window pixels.

Eligibility and fallback

HazeFeatureFlags.isPlatformBackdropEnabled is an experimental process-wide switch. It is false by default. Set it before attaching the Haze effect nodes that should observe the switch:

HazeFeatureFlags.isPlatformBackdropEnabled = true

The switch makes the platform renderer eligible; it does not force native rendering. Built-in effect support, the Android full 37.2 SDK gate, the current window and canvas, native setup, and native drawing must all succeed. Unsupported platforms, incompatible effects, software canvases, and setup or draw failures use source fallback. A known unavailable or failed native path remains sticky until that modifier node detaches. Existing attached nodes keep the value observed at attachment; a later attachment reads the current value.

The current implementation is experimental and has not established physical Android 37.2 acceptance or a performance result. Those remain release gates. The rollout is deliberately staged: after acceptance a later release may enable the default, retain false temporarily as a regression escape hatch, and then remove the flag.

Diagnostics and consequences

Enable HazeLogger.enabled while diagnosing selection and fallback messages. Native draw work is also marked by the HazeBackdrop.draw trace section. These are diagnostics, not a public backend selection or status API.

On a healthy native attachment, Haze does not record dormant source layers. A native-to-source transition may take one frame, and source fallback remains responsible for portable rendering and retained-output behavior. ADR-0009 records the earlier wrapper-based decision and remains unchanged as historical context; this ADR supersedes its public input shape.