Each target's row shows its result as soon as its check ends (closes #91)

tick drew no row until the round's slowest check ended, up to 24
seconds at a 30-second interval since checks time out at 80% of it.
Each check now pushes its sample and redraws its row as it ends;
sorting, the summary, the health box and offline detection still run
once, after the last check. A check that ends while paused or after its
round is given up draws nothing, and the first round is still discarded
as a whole. The row is looked up when the check ends, as a pin click
can re-sort the rows mid-round.

The new unit test runs tick on the mocked clock against a stand-in
page.

Model: opus-5-5

README.md

@@ -104,9 +104,10 @@ code lives in `src/main.js` with a class-based architecture:
   color-coded line segments, error regions, and DPR-aware scaling
 - **UI functions**: `buildUI()` constructs the DOM, `updateHostRow()` /
   `updateSummary()` / `updateHealthBox()` handle incremental updates
-- **`tick()`**: Main loop — measures all hosts in parallel via `Promise.all`,
-  pushes samples, redraws UI. When paused, pushes blank markers (no probes, no
-  false outage)
+- **`tick()`**: Main loop — measures all hosts in parallel, pushing each host's
+  sample and redrawing its row as soon as its check ends, then sorts and redraws
+  the summary and health box once the last check ends. When paused, pushes blank
+  markers (no probes, no false outage)
 - **`Reporter`**: Posts collected samples to the backend
 
 ### Reporting
@@ -144,8 +145,8 @@ have all finished before the next round is due. When no WAN host answers, a
 recovery probe checks 4 random WAN hosts every half second, giving up the checks
 it started half a second before. As soon as one answers, a new round starts at
 once, as it does after an interval change. A round started early gives up the
-last round's checks if they are still waiting, and that round records nothing,
-so rounds never overlap. IPv4 only.
+last round's checks if they are still waiting, and that round records nothing
+more, so rounds never overlap. IPv4 only.
 
 ### Color coding