Mod system: WASM mod loading scaffold (extism sandbox)

[mark-ik]

Goal

Implement the WASM mod loading tier of the ModRegistry using extism — the capability-restricted WASM plugin runtime. This unlocks third-party mod distribution without recompilation.

Scope

• Add extism (1.13.0, BSD-3-Clause) to Cargo.toml
• Implement ModType::Wasm loading path in mod_loader.rs
• Define the host-side capability interface exposed to WASM mods:
  • Read-only graph snapshot access
  • Intent submission via a bounded queue (mods cannot call apply_intents directly)
  • Registry query (ViewerRegistry, PhysicsProfileRegistry, ActionRegistry)
  • Diagnostics channel write (via DiagnosticsRegistry namespace)
• ModManifest validation for WASM mods: provides + requires checked against registered capabilities
• Sandbox invariants: no filesystem access, no network access, no keychain access unless declared and granted
• Hot-reload: WASM mods can be updated without app restart (re-load extism plugin from file)
• Capability denial path emits diagnostics event (not panic)

Non-Goals (Phase 1)

• Physics profile mods (tracked separately in canvas lane)
• Viewer mods (require compositor pass hooks — speculative)
• Cross-mod communication

Crate

extism (BSD-3-Clause) wraps wasmtime internally. BSD-3 is license-compatible. wasmtime is pulled transitively; direct dep not needed unless custom host interface is required beyond extism ABI.

Source Docs

• design_docs/graphshell_docs/implementation_strategy/subsystem_mods/SUBSYSTEM_MODS.md
• design_docs/graphshell_docs/implementation_strategy/system/2026-02-22_registry_layer_plan.md (§ModRegistry)
• design_docs/graphshell_docs/implementation_strategy/canvas/2026-02-24_physics_engine_extensibility_plan.md (extism ABI rationale)
• design_docs/graphshell_docs/technical_architecture/2026-03-01_dependency_inventory.md (extism: 🟢 Adopt when ready)
• design_docs/TERMINOLOGY.md (WASM Mod definition)

Dependencies

• ModRegistry native mod tier already active (inventory::submit! path)
• DiagnosticsRegistry channel schema active
• ActionRegistry routing active

Done Gate

• A minimal WASM mod (hello-world physics profile) loads, executes, and registers via ModRegistry
• Sandbox violation (filesystem open attempt) is caught and emits diagnostics, does not crash
• ModManifest provides/requires validation rejects mods with missing capability declarations
• Hot-reload: mod file can be replaced on disk and reloaded without app restart
• Capability grant/deny flow documented in SUBSYSTEM_MODS.md