Refactor: extract the editor into its own package (@jsvision/editor)

[blendsdk]

Summary

Extract the multi-line editor family (currently packages/ui/src/editor/) out of @jsvision/ui
into a dedicated @jsvision/editor package. This isolates a heavy, optional subsystem, gives
the forthcoming code-grade editor work (RD-08, #18) a clean home, and lets ui-only consumers avoid
carrying an editor they don't use.

Decision made 2026-07-18 (discussion under #18).

Motivation

• The editor is a large, optional subsystem — apps that only want widgets shouldn't carry a code editor.
• RD-08 (RD-08 Editor family: a code-development-grade editor (line numbers, syntax highlighting, folding) — in-house, no CodeMirror dep #18) will add substantial bulk (gutter, syntax highlighting, folding). It is cheaper to move
  the small current editor than the big future one, so this should land before RD-08's build.
• Cleaner layering: ui → editor → lang, where the planned @jsvision/lang (Lezer) depends on the
  editor's seam rather than on all of ui.

Current coupling (measured)

The editor imports from nine ui subsystems — view (11×), reactive (6×), dialog, status,
layout, controls, window, scroll, dropdown — plus two internal-file reaches:
controls/measure.js (stringWidth/glyphWidth) and dialog/message-box.js
(runDialog/messageBox/ModalDialogHost). And @jsvision/files depends on the editor
(FileEditor). So extraction is a real refactor, not a lift-out.

Work

• Create packages/editor/ (@jsvision/editor); move packages/ui/src/editor/** into it.
• Widen @jsvision/ui's public API to export everything the editor consumes — notably
  stringWidth/glyphWidth (measure) and runDialog/messageBox/ModalDialogHost
  (message-box) if not already public.
• Re-point @jsvision/files from ui's editor exports to @jsvision/editor.
• Add @jsvision/editor to the lockstep-version set (sync-package-versions).
• Move the editor's kitchen-sink stories/tests as needed; keep the smoke tests green.
• yarn verify + yarn gate green.

Related decision (feeds RD-08 #18)

• The Tokenizer/FoldProvider seam interfaces will live in @jsvision/core (pure zero-dep type
  declarations), so the future @jsvision/lang (Lezer) depends only on core + @lezer/*, not on
  ui/editor.
• core/ui keep {} runtime deps; the Lezer dependency is isolated in the opt-in @jsvision/lang
  package (mirrors the @jsvision/forms → zod peer-dep precedent).

Sequencing

Recommended to land before the RD-08 build so the new highlighting/folding/gutter code is authored
in @jsvision/editor from the start. Not a hard blocker — RD-08 could be built in ui and
extracted later, at higher cost.

Acceptance criteria

• @jsvision/editor exports the full editor family; @jsvision/ui no longer contains editor source.
• @jsvision/files builds against @jsvision/editor.
• No behavior change; all existing editor tests + kitchen-sink smoke pass.
• yarn verify green; lockstep versions synced.

[blendsdk]

Closing as completed per maintainer direction following the open-backlog audit. Related follow-on issues remain independently tracked where applicable.