Code-grade editor: syntax highlighting + tree-based folding via Lezer (new @jsvision/lang)

[blendsdk]

Summary

Build the code-development-grade editor (line numbers, syntax highlighting, tree-based folding)
on top of the existing @jsvision/ui editor, using Lezer (@lezer/*) as the language engine.

This is the build follow-through for the closed epic #18 ("RD-08 Editor family: a
code-development-grade editor … in-house, no CodeMirror dep"). A 9-probe feasibility spike returned
GO; the spike branch/package were never committed, so the decision now lives in
codeops/features/code-editor/plans/feasibility-spike/decision-memo.md (reconstructed).

Reinterpreting #18's "no CodeMirror dep"

Still holds. Lezer is CodeMirror's incremental parser, not its editor view. We take the parser
(pure JS, headless — no DOM, no WASM, no native; passes check:deps) and reject CodeMirror's
view/language/commands and Monaco. The shipped @jsvision/ui editor is the view.

Architecture (settled)

Three moving parts:

1. Extract the editor @jsvision/ui → @jsvision/editor — tracked in Refactor: extract the editor into its own package (@jsvision/editor) #101, lands
   before this build. Coupling is real: the editor imports 9 ui subsystems + reaches
   controls/measure.js and dialog/message-box.js, and @jsvision/files depends on it.
2. Seam types (Tokenizer / FoldProvider) in @jsvision/core — pure zero-dep declarations,
   so the Lezer package depends only on core + @lezer/* (never on ui/editor).
3. Lezer engine in a new opt-in @jsvision/lang — @lezer/* as regular deps.

core/ui keep {} runtime deps (mirrors @jsvision/forms → zod). The indent-based default
folder + gutter live in @jsvision/editor (zero-dep), so basic folding works without Lezer.

Lezer only — no TextMate fallback. It is Lezer or we don't build this. Accepted consequence: the
supported-language set = languages that have (or for which we author) a Lezer grammar; a language
with no Lezer grammar is out of scope until one exists (no regex/TextMate second path).

Work

• Land Refactor: extract the editor into its own package (@jsvision/editor) #101 (extract @jsvision/editor) — prerequisite.
• Add Tokenizer / FoldProvider seam types to @jsvision/core.
• Scaffold @jsvision/lang (@lezer/* deps); implement the (from,to,tag) Tokenizer +
  FoldProvider for one language end-to-end.
• Gutter + indent-based default folder in @jsvision/editor (works without @jsvision/lang).
• Bounded-chunk buffer → Lezer Input adapter; off-frame/time-budgeted parse for large files.
• Per-language fold allow-lists (~8 node names/language).
• make_requirements RD-08 with the decision-memo as settled evidence.
• Kitchen-sink story + headless smoke test for the highlighted/foldable editor.
• yarn verify + check:deps stays clean (core/ui remain zero-dep).

Not in scope / re-measure

Per-probe timings and exact @lezer/* versions from the original spike are unrecoverable (lost
with the uncommitted spike). Re-spike only if a specific number becomes load-bearing.

Related

• Epic (closed): RD-08 Editor family: a code-development-grade editor (line numbers, syntax highlighting, folding) — in-house, no CodeMirror dep #18
• Prerequisite refactor: Refactor: extract the editor into its own package (@jsvision/editor) #101
• Decision memo: codeops/features/code-editor/plans/feasibility-spike/decision-memo.md
• Separate pre-existing bug to file: format.ts sums ZWJ/multi-codepoint emoji width (>2) while
  clamping the cell to 1|2.

[blendsdk]

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