docs: record :sync latency budget from hardware measurement

New docs/notes/sync-latency.md breaks down the measured ~16 s cold :sync
(Wi-Fi + SNTP + one TLS push), explains the optimistic-retry handshake saving,
the reconcile/last-writer-wins semantics, and why the rest is near the protocol
floor. Linked from the notes index, the docs index, and the Git row of the root
README.

docs/notes/README.md

@@ -11,3 +11,4 @@
 | [`ctrl-g-perceived-latency.md`](ctrl-g-perceived-latency.md) | Durability before delivery — surfacing "commit landed" at ~0.2 s makes the 5–10 s `Ctrl-G` push feel instant. |
 | [`git-sync-images-and-repo-size.md`](git-sync-images-and-repo-size.md) | Why we don't shrink the notes repo — its 153 MB of media is remanso's image CDN, so rewriting history to slim the on-device clone breaks the web app. |
 | [`boot-time-budget.md`](boot-time-budget.md) | Where the ~4.3 s to cursor goes — and why the ≤ 3 s v1.0 target is hard: one ~1.9 s full refresh is unavoidable at cold boot, so the splash is nearly free. |
+| [`sync-latency.md`](sync-latency.md) | Where the ~16 s cold `:sync` goes — Wi-Fi + SNTP + one TLS push; why optimistic-retry cut a whole handshake, and why the rest is close to the protocol floor. |