[ryanbr]

Summary

Stacked on @andigandhi's #2445. Both of their commits are preserved with their authorship and messages unchanged.

Their change gives the Deep Timeline x-axis labels and per-minute ticks at deep zoom, on Android and Apple, with the tick positions shared between the platforms and the gridlines and label strip driven from one tick list so they cannot drift. The Android half also pins three new tiers in ChartTimeTicksTest.

The follow-up commit

chartTimeTicks is shared with the Today HR card. The Deep Timeline hands it a pinch-zoom window, but Today hands it visBuckets.first().bucket … visBuckets.last().bucket, the RENDERED extent of banked buckets rather than a nominal window. So tiers below an hour reached Today too: a morning holding ten minutes of HR drew ten 1-minute dotted gridlines on a small card. Labels are thinned by an overlap skip, the gridlines are not. The sub-hour tiers now sit behind deepZoom, off by default and set only by the surface that zooms, which also makes the retained comment about thresholds sitting below the nominal Today windows true again.

Swift keys that on zoomBounds, not zoomDomain, for the reason OverviewHRChart already records twice: zoomBounds is set once by the zooming call sites, while zoomDomain stays nil until the first pinch, so keying on it would hand a not-yet-zoomed Deep Timeline the static tiers and then switch them under the user (#829).

The Swift function formatted labels nothing read. xTicks discarded every one of them with .map { $0.date }, while the function built a fresh DateFormatter per call. xTicks is read inside body, so during a pinch that is a formatter construction plus N string formats per frame, all thrown away. It now returns dates and formats nothing, and its doc no longer claims the platforms read identically: positions are shared, labels are not, since Swift draws them with AxisValueLabel() in the viewer's locale where Kotlin formats HH:mm itself.

Also worth stating plainly for whoever reads this later: because OverviewHRChart is shared, this PR changes the Today HR card's Apple axis from .automatic(desiredCount: 5) to round wall-clock ticks. That is a deliberate keep, since Android's Today card already ticks this way, so it closes a platform difference rather than opening one.

Verification

• ./gradlew :app:testFullDebugUnitTest --no-build-cache --rerun-tasks: 760 suites, 6479 tests, 0 failures, 0 errors. ChartTimeTicksTest is 13 of 13.
• python3 Tools/doc_comment_lint.py: no new detached doc comments.
• ChartTimeTicksTests is new on the Swift side, pinning all eight tiers, the Today guarantee and an inverted window. Eight thresholds written twice in two languages drift, and a drift is invisible in a build. It runs on the test (StrandDesign) leg.

Refs #575