test(ui): pin the _user_adjusted resize guard both ways; fix coalescing doc overstatement (PR #188 review)

Hoshe's finding: the flag introduced so auto-fit never fights a manual
view had zero coverage on exactly that branch. Two tests drive the
REAL _gui_input path (synthetic drag), then fire NOTIFICATION_RESIZED:
user-adjusted view survives a resize untouched (zoom AND offset);
an unadjusted view re-fits to the new viewport. 52/52.

Non-blocking doc note also taken: layer_proxy's normalization comment
claimed the twins would otherwise 'coalesce independently' — the
coalescing key is (ConnectionId, body_id) and never carried center;
rewritten to say what normalization actually buys on that path (the
work item derives and echoes the canonical center).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/atlas/layer_proxy.rs

@@ -821,9 +821,13 @@ fn normalize_window_center(params: &BodyParams, center: DistrictPos) -> District
 /// rather than only inside the former miss-branch: normalization needs
 /// `body_radius_km` to compute the wrap/clamp bounds, and it must happen
 /// before either key exists, or an insane request and its sane normalized
-/// twin would land in different cache entries / coalesce independently
-/// (exactly the bug this fix closes — a garbage `window_center` was cached
-/// standalone instead of collapsing onto its valid twin). The one-time cost
+/// twin would land in different cache entries (exactly the bug this fix
+/// closes — a garbage `window_center` was cached standalone instead of
+/// collapsing onto its valid twin). The coalescing key itself is
+/// `(ConnectionId, body_id)` — it never carried `center`, so coalescing
+/// was never at risk of diverging per-center; normalizing before
+/// `submit_window` matters only so the work item DERIVES (and echoes) the
+/// canonical center. The one-time cost
 /// (a single indexed `bodies` row read) is paid on every window request now,
 /// not just on a cache miss — a request whose normalized center hits the
 /// cache still needed this read to know WHICH key to check.