fresh-ui: no command-palette primitive (⌘K); Combobox is not sufficient

[rickylabs]

Summary

@netscript/fresh-ui ships no command-palette primitive. A ⌘K surface is now
standard furniture in this class of app, and building one correctly is more
subtle than it looks — I have just done it by hand in eis-chat and would
rather contribute it back than have the next app repeat the work.

What is missing

Combobox is close but not sufficient. It is a headless roving-keys input
over a listbox, which gives you the input and the keyboard, but a command
palette additionally needs:

• grouped results with headings, where arrow keys traverse the flattened
  order across group boundaries;
• filtering and ranking owned by the consumer — Combobox does not filter,
  which is correct, but every consumer then reimplements the same fuzzy scorer;
• a virtual active row via aria-activedescendant with rows as
  role="option" that are not tab stops, so the surface has exactly one
  focusable element;
• mousemove-driven hover rather than mouseenter, or the keyboard
  highlight is stolen whenever the list scrolls under a stationary pointer;
• an empty state that is a peer of the groups rather than a listbox child;
• Escape as a two-stage retreat (clear the query, then close).

Composing Dialog + Combobox also currently inherits #1688.

Proposed shape

A headless useCommandPalette plus a thin Command.* namespace mirroring the
parts that have become conventional (Root, Input, List, Empty, Group,
Item, Separator, Shortcut), with:

• items: readonly {id, group}[] in display order, and the hook owning
  activeIndex, wrap-around, Home/End/PageUp/PageDown, and
  scrollIntoView({block: 'nearest'});
• getInputProps() emitting role="combobox", aria-expanded,
  aria-controls, aria-activedescendant, aria-autocomplete="list";
• getItemProps(index) emitting role="option", aria-selected, the option
  id, and the mousemove handler;
• ranking left to the consumer, with an optional exported subsequence scorer.

Prior art to copy from

eis-chat's implementation, which is tested and measured:
https://github.com/rickylabs/eis-chat/pull/255

• lib/commands/score.ts — fuzzy subsequence scorer (DP over query x text)
  with word-start / acronym / consecutive-run bonuses and gap penalties,
  returning match ranges for highlighting. 10 unit tests.
• lib/commands/search.ts — grouping, per-group browse caps, group ordering by
  best member, and a "breadcrumb only when it is not an echo of its own group
  heading" rule.
• lib/commands/registry.ts — provider registry so features contribute
  commands without editing the palette.
• components/ui/command-palette.tsx — the a11y wiring described above.

Happy to raise a PR extracting the headless half if the shape looks right.