Vox playground architecture (research)

Scope: Architecture-only — no playground runtime ships with this note. It folds Phase 4 monitor ideas (vox-language-rules-phase4-runtime-monitors-2026.md) into a concrete product surface.

Problem statement

Authors today rely on:

Surface	Strength	Gap
`vox check` / `vox fmt`	Full compiler pipeline	Not browser-local; no embeddable UX.
`vox shell repl`	Fast feedback loop	Not a structured teaching surface; limited linkage to diagnostics / fixes.
IDEs via `vox lsp`	Rich editing	Requires install; blocks zero-install tutorials.

A playground closes the zero-install loop while staying inside telemetry + sandbox policy (telemetry-trust-ssot.md).

Syntax & diagnostics tier — Wasm bundle of lexer/parser/typecheck read-only stages (mirrors crates/vox-compiler algorithms). Emits the same structured payloads as VoxCompilerDiagnosticPayload / vox check --for-llm.
Formatter tier — Optional second Wasm module wrapping vox_compiler::fmt::format; must respect idempotency tests (format_round_trip.rs).
Execution tier (optional) — Behind explicit Run consent: vox-actor-runtime wasm/isolation lane (vox run --isolation wasm semantics), fuel + allocation caps per Phase 4 plan. No ambient filesystem writes — vox-bounded-fs only.
Telemetry tier — Default local-only mirrors CLI trust boundaries; remote upload remains opt-in per ADR-023. Planned vox.idiom.* families are specified in contracts/telemetry/idiom-events.v1.yaml.

Share URL encodes compressed source + syntax_version (never secrets).
Diff pane shows structured fixes coming from LSP-shaped JSON (parity with IDE quick-fixes).
Deterministic mode exposes --seed contracts from Phase 4 (vox playground --deterministic) once wired.

Hosting arbitrary network calls from the playground Wasm tier (would violate effect policy until Phase 5 UX exists).
Replacing vox shell repl — playground optimizes documentation + teaching, not power-user shell ergonomics.

When implementation lands, require:

Snapshot parity tests mirroring diagnostic_snapshots / vox-lsp/tests/diagnostic_snapshots.rs.
vox ci parse-status alignment for golden corpus examples (examples/PARSE_STATUS.md).