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>

docs-site/README.md

@@ -3,9 +3,9 @@
 The punktfunk documentation site: [Fumadocs](https://fumadocs.dev) on
 [TanStack Start](https://tanstack.com/start) (Vite + Nitro/bun preset).
 
-Content lives in [`content/docs/`](content/docs) as `.md`/`.mdx`. Several pages are imported
-verbatim from the repo's `docs/` design notes (with added frontmatter); edit those there or
-here as the docs site becomes the source of truth.
+Content lives in [`content/docs/`](content/docs) as `.md`/`.mdx`. This site is the source of truth
+for the **user-facing** guides; repo-internal design rationale lives in
+[`../design/`](../design/README.md).
 
 ## API reference