docs: rework client/crate READMEs, add missing ones

Rework the client READMEs to be accurate and inviting to first-time
visitors, and fill in the gaps where crates and tools had none.

- Rewrite clients/{apple,android,decky} READMEs (features-first, trim
  dense internal narrative; drop the stale "one session at a time" /
  "renegotiation not implemented" section from the Apple README).
- Add READMEs for clients/{linux,windows,probe}, which had none.
- Add crate READMEs for punktfunk-host, punktfunk-core, pf-driver-proto.
- Add brief READMEs for tools/{loss-harness,latency-probe}.
- Fix packaging/README duplicate "Option B" heading (bootc -> Option C).
- Fix docs-site/README stale docs/ -> design/ reference.
- De-stale packaging/windows/drivers/pf-dualsense README (drop "M0 spike"
  / external-checkout framing; reflect in-tree workspace + shipped +
  installer-bundled + multi-pad), keeping the driver-authoring lore.

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

tools/latency-probe/README.md

@@ -0,0 +1,15 @@
+# latency-probe
+
+A **glass-to-glass latency** measurement tool (design/implementation-plan §10): it renders a
+timestamp/QR on the host, reads it back off the client's capture (or a photodiode, for true photons),
+and tracks p50/p99 — so latency regressions are quantifiable rather than felt.
+
+> **Status: scaffold.** The measurement harness is stubbed out and not yet wired to a live pipeline.
+> For latency numbers today, use the per-frame **capture→…→reassembled** percentiles the
+> [`probe`](../../clients/probe/README.md) client reports over a real `punktfunk/1` session.
+
+```sh
+cargo run -p latency-probe        # from the repo root
+```
+
+Companion to [`loss-harness`](../loss-harness/README.md) (FEC loss sweep).