fix(explorer): responsive bar, wider legend, 3D domain names, smooth type toggles, no stranded terms

[CMaintz]

Owner feedback (2560x1440) on the Explorer, plus the "no stranded nodes" fix. Decision note A93b (amends A93a).

1. Responsive control bar. The bar measures the space it has (a ResizeObserver on its slot, then a before-paint check of its in-flow width, so an open popover or result list never shrinks it). It steps down from full (domain pills with names, the Find a term field open, "Relationships") to medium (dot chips), then compact (the old "Links" plus a search icon), then the Controls sheet. It never wraps to a second row, and it never overlapped the legend or the "i" at any size tested. Measured sizes: 2560 and 1440 full, 1280 with the legend closed full and with it open medium, 1100 medium, below ~900 the sheet.
2. Bar follows the theme. It already used tokens (bg/surface/border/fg), and so do the popovers. This was verified in light and dark screenshots.
3. Cream void. This was a stale cache (owner confirmed). Checked in 2D and 3D in light theme, on first load and when switching live.
4. Legend sizing. Collapsed, it is at least 10rem wide from 1440px up. Open, it is clamp(18rem, 22vw, 28rem). The bar's left inset follows it (--xp-legend[-open]).
5. 3D domain names. A large, faint name sprite sits above each galaxy in the domain's colour. It is hidden with its domain, dims when a term is selected, hovered or on a route, and follows the theme. It never takes a click.
6. Toggle hitching. In 2D, edge classes are worked out in plain data and only the edges that change are touched. More than 120 changes are applied a batch per frame; a smaller change fades in. In 3D, a type or "show all" toggle skips re-evaluating 3d-force-graph's accessors, and the web appears a batch per frame. Reduced motion makes it all instant.
B. No stranded terms. backboneOf(..., visible) is computed over the shown terms in both 2D and 3D. With AI off, Data poisoning keeps its strongest visible edge. There is a unit test for this.

Longest frame at 2x CPU throttle, 1440x900 (two runs):

toggle	before	after
2D show all on	700-735 ms	150 ms
2D show all off	420-585 ms	117-134 ms
2D a type on	280-550 ms	100-134 ms
3D show all on	430-820 ms	134-150 ms
3D a type on	230-500 ms	117-200 ms
Idle, pan, zoom, orbit and hover fps are unchanged (perf.mjs, 2x throttle).

Screenshots at 2560x1440, 1440x900 and 1280x800, in light and dark, 2D/3D/legend open, plus a live theme switch, are in the agent scratchpad under perf/ux/after, perf/ux/check and perf/ux/before.

mise run fix && mise run gate exits 0. Merged with origin/main (#66, #67). Not merging: the owner merges.