Dock the HUD: every tool and readout on one rail along the bottom

The overlay had grown by accretion: a search box floating top-centre, a
readout panel bottom-left, a range readout bottom-right, and nothing that
said these were parts of one instrument. This puts them on one rail across
the bottom of the viewport — the dock — with a tab strip pinned to the bottom
edge and whichever panel is open growing upward from it. The top of the
screen keeps only the scale ladder and the nameplate, so the map itself is
what fills the frame.

Three tabs. SEARCH is the old search, with its field pinned to the bottom of
the panel and the results growing upward above it, so the thing being typed
into never moves while the list grows. READOUT is the old bottom-left panel.
DISPLAY is new: five layer toggles — labels, orbits, grid, deep sky, sky —
each a real scene object switched by visibility, except the ones the galaxy
crossfade already rewrites every frame, whose toggles fold into that
crossfade instead of fighting it. The range readout sits on the strip itself,
so it is readable whatever is open.

Behaviour worth stating: choosing a search result hands the panel straight
back to the readout, since the thing to look at is now the scene. `/` opens
the search from anywhere. Below `sm` the dock is the strip alone; a tap opens
a panel as a sheet, a tap on the scene folds it away. The body-detail page
gets the same dock with only the search — the info panel is its reading.

Two things found on the way. CSS2DRenderer gives every label its own
z-index for depth order, and the label host created no stacking context, so
labels painted over every HUD panel; `isolate` on the host keeps them under.
And starmap-hud's readout tests were really tests of the panel that moved,
so they moved with it.

Verified: build clean, 535/535 unit, 6/6 end-to-end, design detector clean,
screenshots at 1440×900 and 390×844 across galaxy, galactic, system,
body-detail, all three tabs and the layers-off state.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016jxMkwA2rbicdGxHosecYi

e2e/galaxy-view.spec.ts

@@ -10,7 +10,8 @@ test.describe('Galaxy view', () => {
     await page.goto('/?stars=4000');
 
     await expect(page.getByTestId('scene-canvas')).toBeVisible();
-    await expect(page.getByPlaceholder('Search stars, planets, exoplanets…')).toBeVisible();
+    // The dock's tab strip is up before the scene finishes booting; the search is one tab in it.
+    await expect(page.getByRole('tab', { name: 'Search' })).toBeVisible();
     await expect(backButtonLocator(page)).toHaveCount(0);
     // The readout panel's own title, not just the text anywhere on screen: the selected-object
     // banner across the top names the same thing, so a bare text match is ambiguous.