Debug log adds each line instead of drawing the whole panel (closes #131)

With the debug log open, every line written drew the panel again with
up to 1000 lines, and a check that ends while the page draws is timed
only when the drawing is done, so latencies measured with the panel open
read too high. Each line is now appended to the open panel and its first
line dropped once it holds more than maxLogEntries; the whole panel is
drawn only when it is opened. Scrolling to the newest line reads the
panel's height, which lays out all its lines, so it runs once a frame,
with at most one frame request waiting.

Model: opus-5-5

TODO.md

@@ -21,6 +21,15 @@ Run `make frontend-viewport-test` in CI as its own step; it is not part of
 
 # Completed Steps
 
+- 2026-10-14: opening the debug log no longer raises the measured latencies
+  ([#131](https://git.eeqj.de/sneak/netwatch/issues/131)). With the panel open,
+  each line written drew the panel again with every line before it, up to 1000,
+  and a check that ends while the page draws is timed only when the drawing is
+  done. Each line is now added at the end of the open panel, its first line is
+  dropped once it holds more than 1000, and it scrolls to its newest line once a
+  frame rather than once a line; the whole panel is drawn only when it is
+  opened. A unit test writes more lines than the panel keeps and checks that the
+  lines already shown are left as they are
 - 2026-10-14: the repo has the layout `REPO_POLICIES.md` gives
   ([#30](https://git.eeqj.de/sneak/netwatch/issues/30)). The Go module is at the
   root (`go.mod`, `go.sum`, `.golangci.yml`, `cmd/netwatch-server/`,