docs(design): trim shipped plans, consolidate cluster, add index

Much of design/ described work that has since shipped. Trim each doc to
its durable rationale + still-open items (the code is the source of truth
for shipped detail; git history holds the full originals).

- Shipped plans -> status stubs: stats-capture, gamestream-host-plan,
  apple-stage2-presenter, windows-service.
- Trimmed completed-out / open-kept: implementation-plan, hdr-pipeline,
  host-latency, gpu-contention (fixed stale status table), game-library,
  linux-setup (fixed m0->spike + stale zero-copy claim),
  session-aware-host-followups, windows-client-bootstrap,
  windows-dualsense-{scoping,game-detection}, windows-virtual-display,
  security-review (per-finding status table; #12 still open),
  apollo-comparison (shipped backlog collapsed to one-liners).
- Windows-host cluster consolidated: windows-host.md -> redirect into
  windows-host-rewrite.md (whose stale scorecard is corrected -- goal1 is
  merged, M4 done); windows-secure-desktop.md archived (now a fallback
  behind IDD-push primary).
- Kept evergreen: ci.md, gamescope-multiuser.md, windows-build-and-packaging.md.
- New design/README.md: per-doc status table + consolidated open-items
  roll-up so nothing is tracked in only one buried doc.
- Repoint 5 code comments to the archived secure-desktop doc path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

crates/punktfunk-host/src/punktfunk1.rs

@@ -2364,7 +2364,7 @@ fn virtual_stream(ctx: SessionContext) -> Result<()> {
     // Windows two-process secure-desktop path: when the host runs as SYSTEM (required for the secure
     // desktop + SendInput), WGC can't activate in-process, so we capture the normal desktop via a
     // helper spawned in the user session and relay its AUs. (Single-process WGC/DDA is used as the
-    // user, and stays the path on Linux.) See design/windows-secure-desktop.md.
+    // user, and stays the path on Linux.) See design/archive/windows-secure-desktop.md.
     #[cfg(target_os = "windows")]
     if plan.topology == crate::session_plan::SessionTopology::TwoProcessRelay {
         return virtual_stream_relay(ctx);