Draft framing for the folding arc, committed to the `folding` branch for
review before any implementation. Branch is cut from canonical main @
cac4961 (post Vterm Stage 3 #135).
The load-bearing finding: the bundled tree-sitter grammars ship no fold
query and no folds.scm — they export HIGHLIGHTS/INJECTIONS/LOCALS/TAGS
only. The roadmap's "tree-sitter fold ranges" premise is therefore not
free, so the fold source is a real decision (Q#FD1). The draft recommends
structural node folding (fold the nearest enclosing block-like node
spanning >= 2 rows), which reuses the existing parse trees for every
grammar and injection layer with zero per-language authoring; indentation
folding (grammarless fallback) and curated per-language queries (quality
pass) are deferred.
FoldState already exists in the protocol, declared but unproduced, with a
test pinning that it is never emitted; no frontend consumes it; gutter
markers are frontend-derived like the diagnostic sign bars, so no new wire
type is needed. Staged like vterm: Stage 1 engine (instance-side fold
model + structural source + Lua commands + FoldState production, headless),
Stage 2 TUI collapse+gutter, Stage 3 GPU at parity.
Numbered decisions Q#FD1-9, three falsifiable bets, named deferrals, and a
Stage 1 acceptance list. Awaiting review rounds; bindings (Q#FD4) and the
block-kind heuristic (Bet B1) are the two calls flagged for the user.
