Let a short window fold the HUD's scale and timeline away

[zkWizard]

The decision three backlog items were waiting on

docs/BACKLOG.md carries three open UI/UX items — the HUD covering the aim
below ~610px, a landscape phone reaching none of the nine overlays, and a phone
dropping the layer caption — and all three end at the same sentence: "a
collapsible or compact panel below a size threshold. Three items now wait on
that one call." #980 spent the panel's spacing buying the aim back and said in
its own comment that the spacing was then spent; what is left can only come out
of content.

This makes that call in the one way that does not require guessing on the
reader's behalf: the panel keeps every row it has ever had, and gains a way to
fold the two tall ones out of the view. Nothing folds on its own.

Visible: on a window 720px tall or shorter, a chevron appears in the HUD
panel's top-right corner. Pressing it folds the colour scale and the timeline
away — the panel drops from 294px to 75px at 1280x620 — leaving the layer
selector and the provenance line. Pressing it again restores them. Above 720px
tall the control is not rendered at all.

What the fold keeps, and why that is the design

The layer selector still names what is on the globe, and the provenance line
still carries the product ID and the month (MODIS_Terra_L3_NDVI_Monthly · Jul 2026). No citation and no date is lost to a gesture meant to buy screen
space. What goes is the colour ramp and the timeline — which are the height,
and which the reader has just said they are not reading. This is deliberately
unlike the phone-caption item the backlog files separately: there the app drops
a compositing caveat silently at a width the reader never chose, here nothing
goes without the gesture that asks for it. No scientific wording was moved or
reworded by this PR.

Verified

• Height-neutral, measured rather than asserted. The button is out of flow
  and the corner it reserves is horizontal. Re-measuring the expanded panel with
  my two rules neutralised at runtime gives the identical number — 294 vs 294
  at 1280x720 and 266 vs 266 at 1280x665. This is what stops the affordance
  from pushing the panel back over the aim it exists to uncover, and it is a
  test, not a note.
• The landscape win, stated as the defect. At 844x390 the spec first asserts
  the centre of the view is not the globe — the panel is three quarters of the
  display — then folds the panel and polls until it is. That premise passed on
  origin/main during the red-check, so the defect is real and not an artefact
  of the spec.
• Cannot strand a row. Both the button and the collapsed state are declared
  inside the same @media (max-height: 720px) block, so a window growing past
  the threshold restores every row on its own. Without this a reader could fold
  the panel at 600px tall, maximise, and be left with rows hidden and no visible
  control to bring them back. The last test drives exactly that sequence.
• The control meets the 44px touch target on the device it exists for. Its
  first draft was 28px everywhere, which is under the app's own coarse-pointer
  standard on exactly the case this PR headlines — a phone in landscape. It now
  takes 44px under (max-height: 720px) and (pointer: coarse), and the corner
  reserved for it grows with it so the bigger box cannot land on the layer
  selector. This could not be added to a11y.spec.ts's TOUCH_TARGETS list:
  that suite runs at 390x844, a height where this button is deliberately not
  rendered, and it counts "not rendered" as a failure — so the guarantee is
  pinned in the new spec at a size where it does render. No existing spec is
  both short and coarse, so the block is reachable only by the new assertion.
• Red-checked: with index.html, src/style.css and src/main.ts reverted
  to origin/main, 3 of the 4 new tests fail for the right reason (the control
  does not exist). The fourth passes by design — it asserts the control is
  absent at roomy heights, so it is a guard rather than a red-check candidate.
• Full local chromium e2e run for the layout change (the HUD trap); unit suite
  4755 passed / 315 files; tsc and prettier clean.
• Bundle: entry chunk 51.7 kB gzip ok (from 51.1) — one button, two inline
  chevrons and a toggle. Every other chunk ok.

Not closed by this PR

The affordance shipped alone; none of the three items is checked off, and
docs/BACKLOG.md records what each still needs now that the mechanism exists —
the landscape item in particular is unblocked rather than fixed, since the
nine overlays stay unreachable until the bottom-bar layout is re-measured
against a folded panel.

Draft #959 also touches index.html and src/style.css; it is a parked
maintainer decision and was not modified here.

🤖 Generated with Claude Code