feat(render): add output-specific DMA-BUF feedback (#53)

Co-authored-by: Samuel Onoja <[email protected]>

docs/rendering-performance.md

@@ -86,5 +86,5 @@ actual framebuffer recapture counts/cost, asynchronous capture encoding and
 window-capture readback separately. Current effect metrics are placeholders;
 `capture_us` covers monitor capture batches only. Exact cursor-only KMS updates
 and detailed scanout rejection causes require additional upstream observability.
-Output/per-surface DMA-BUF feedback and eligibility expansion remain later phases,
-with broader primary and overlay policies validated separately on hardware.
+See [DMA-BUF feedback](dmabuf-feedback.md) for the current allocation hints.
+Broader primary-plane and overlay policies still need hardware validation.