design(host): add vrr design doc

design/README.md

@@ -24,6 +24,7 @@ holds the full originals.
 | [`multi-user-profiles.md`](multi-user-profiles.md) | Multi-user / profiles end to end: map a client to a real host OS user account (own isolated desktop), web-console config, per-profile passcode | **Design, schema-of-record** — not yet implemented |
 | [`host-latency-plan.md`](host-latency-plan.md) | Latency under GPU contention — 4-tier plan | **Partly shipped** — superseded by ↓; diagnostics + open tiers kept |
 | [`gpu-contention-investigation.md`](gpu-contention-investigation.md) | GPU-contention root-cause + ranked levers (supersedes ↑) | **Active plan** — §5.A shipped; §5.B/C/E/F/G open |
+| [`vrr-plan.md`](vrr-plan.md) | VRR over punktfunk/1 — skip the VRR virtual display (client panel follows stream cadence; virtual display Hz = sampling grid); frame-driven host loop + VFR rate control + present-on-arrival clients | **Design** — not yet implemented |
 | [`hdr-pipeline-plan.md`](hdr-pipeline-plan.md) | Glass-to-glass HDR | **Steps 0–3 shipped**; Step 4 (Linux) open |
 | [`windows-host-rewrite.md`](windows-host-rewrite.md) | **Windows host — the single architecture/status/reference doc** (validated invariants, ops, open work) | **Active reference** |
 | [`windows-build-and-packaging.md`](windows-build-and-packaging.md) | How the Windows host is built, signed, packaged (drivers-from-source, Inno, CI) | **Evergreen reference** |
@@ -53,6 +54,7 @@ owning doc.)
 - Sub-frame pipelining — overlap encode+transmit within a frame; needs a direct NVENC SDK wrapper (~2–4 ms). → `implementation-plan`, `gamestream-host-plan`
 - GPU-contention levers: correct async NVENC pipeline, auto-gated REALTIME GPU priority, clock/P-state pinning, frame-source escape (swapchain-hook/NvFBC/compose-flip), iGPU encode offload, PERF uniq-vs-fps instrumentation. → `gpu-contention-investigation` (§5.B/C/E/F/G), `host-latency-plan` (Tiers 1A/1B/3B/3C/3D/4)
 - Apple stage-2 as default (after resolution/HDR checks) + smoothing/pacing policy + glass-to-glass numbers via `tools/latency-probe`. → `apple-stage2-presenter`
+- VRR / frame-driven cadence: Stage A client present-on-arrival (Android/Linux → iOS → Windows), Stage B host frame-driven loop + VFR rate control + `FLAG_REPEAT`/`VIDEO_CAP_VRR`, Stage C gamescope `--adaptive-sync` headless experiment. → `vrr-plan`
 
 **HDR**
 - Linux 10-bit HDR (Step 4): 8-bit→Main10 shim, true 10-bit PipeWire capture (blocked upstream — gamescope #2126), Linux-client P010 + GTK color management. → `hdr-pipeline-plan`