vision: sandbox substrate spike — in-memory RepoSnapshot fork + split pane (trunk)

[Zychs]

Vision

Split-view sandboxing: primary workspace + secondary experimental context; sandbox is a natural extension of the visualizer (README.md Split-View Sandboxing). GHM2 import frames this as the GMS2 build/run/debug → probe/playtest loop.

Reality

• Doc-only, zero code in src/ (ARCHITECTURE: deferred doc-only).
• Spec once named assets/split-sandbox.js — file does not exist.
• Earlier drafts cited fictional suite constructs (derivation-priority-guardian, INDEX junctions) — dropped in Phase A docs; real primitive only: in-memory RepoSnapshot fork (worktrees later optional).

Spike (smallest real thing)

1. Fork a RepoSnapshot in memory.
2. Split pane (primary | sandbox).
3. Apply = pure view-merge, writes nothing to disk.
4. Discard = bit-identical primary + clean git status (VERSIONING §7.2).

Exit

• Fork / probe / view-merge Apply / Discard work.
• Split cannot close without an explicit gate action (see gate issue).
• Unlocks Phase D and verb settlement.

Refs

docs/ghm2-development-plan.md Phase C · frontier array lane 1 [5/5] Trunk · docs/VERSIONING.md §3–4, §7.2