Phase 12: add wasm_v2 scaffold (unified vtable slots), expand host by-slot (Map/String), STRICT extern diagnostics, identical-exec console.log test, and CLAUDE_WASM_TASK handoff

CLAUDE_WASM_TASK.md

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+Title: WASM Backend v2 (Phase 12) – Minimal Handoff
+
+Goal
+- Implement a new `src/backend/wasm_v2/` backend that aligns with the unified vtable/slot dispatch model, without touching MIR-level semantics.
+
+Scope
+- Allowed:
+  - `src/backend/wasm_v2/*` (mod.rs, unified_dispatch.rs, vtable_codegen.rs)
+  - `src/backend/wasm/*` read-only for reference
+  - `projects/nyash-wasm/*` (demo HTML/JS harness)
+  - Documentation under `docs/backend/wasm/*.md`
+- Avoid (to keep merge easy):
+  - MIR layer (no plugin-name hardcoding)
+  - `src/runtime/*` core (HostHandle/host_api/extern are assumed stable)
+  - JIT/VM files unless behind feature/cfg guards
+
+Design Constraints
+- ExternCall and BoxCall remain separate at the MIR/semantics level.
+- At lower layers, converge onto a unified host-call shim when needed (same C symbols as VM/JIT).
+- Console/print: use `ExternCall(env.console.log)`; do not move to BoxCall.
+- vtable/slots must align with `src/runtime/type_registry.rs` (see Array/Map/String slots).
+
+Targets & Flags
+- Target: `wasm32-unknown-unknown`
+- Feature: `--features wasm-backend`
+- Use `#[cfg(feature = "wasm-backend")]` and/or `#[cfg(target_arch = "wasm32")]` to isolate new code.
+
+Acceptance (Minimal)
+- Build succeeds for non-wasm targets (no regressions).
+- wasm_v2 compiles behind `--features wasm-backend` (even as stubs).
+- A demo harness can call `env.console.log` from WASM and produce a message.
+- (Nice to have) Array/Map len/size/has/get stubs go through unified dispatch.
+
+Next Steps (Optional)
+- Implement unified dispatch bridge to JS host for `env.console` and basic collections.
+- Add a minimal test or demo comparing VM vs WASM return values for a simple program.
+