fix(codegen-deno): WASI stub in __as_wasmInstance — instantiate affinescript-produced wasm (#534)

## What

The Deno-ESM backend's `wasmInstance` lowering (`__as_wasmInstance` in
`lib/codegen_deno.ml`) instantiated with **no import object**:

```js
new WebAssembly.Instance(new WebAssembly.Module(bytes)).exports;
```

But every affinescript-produced wasm imports
`wasi_snapshot_preview1.fd_write`
**unconditionally** (even a trivial pure-integer module with no
print/panic
path), so the import-less call throws `WebAssembly.Instance(): Imports
argument
must be present and must be an object`. Net effect: the Deno-ESM backend
could
not load *any* affinescript-compiled wasm. The Node backend
(`codegen_node.ml:251-273`) already wires this stub — this gives the
Deno-ESM
backend the same one line:

```diff
 const __as_wasmInstance = (bytes) =>
-  new WebAssembly.Instance(new WebAssembly.Module(bytes)).exports;
+  new WebAssembly.Instance(new WebAssembly.Module(bytes),
+    { wasi_snapshot_preview1: { fd_write: () => 0 } }).exports;
```

## Regression test

`tests/codegen-deno/wasm_wasi_instance.{affine,harness.mjs}` — the
harness
hand-builds an 86-byte wasm that **imports**
`wasi_snapshot_preview1.fd_write`
(declared, never called) and exports `add(i32,i32)->i32`, then drives it
through
the `wasmInstance` lowering. It passes **only** when the stub is present
(without
it, instantiation throws). Mirrors the existing `wasm_call` fixture
(#414), which
used an import-free module and so never exercised this path.

## Validation

Built from source on this branch (`dune build`),
`./tools/run_codegen_deno_tests.sh`
green including the new fixture; `wasm_wasi_instance.harness.mjs OK`.
Separately
confirmed the bytes instantiate **with** the stub (`add(2,3)=5`) and
throw
**without** it.

## Why it matters

Unblocks the Deno-ESM differential-test path downstream — e.g.
`hyperpolymath/idaptik`'s evacuation of its ~87 `*_diff.ts` parity
harnesses to
AffineScript, which load affinescript-compiled co-processor wasm via
this
backend. (Verified end-to-end there: a from-source build with this fix
runs an
AffineScript harness GREEN 1309/1309.)

## Caveat

⚠️ Commit is **DCO signed-off but not GPG-verified** (no signing key in
the
remote execution container). Re-sign locally before merge if the branch
rule
requires it.

https://claude.ai/code/session_01PhqGcxCqkMdJtR6NWq56Hx

---
_Generated by [Claude
Code](https://claude.ai/code/session_01PhqGcxCqkMdJtR6NWq56Hx)_

Signed-off-by: hyperpolymath <6759885+hyperpolymath@users.noreply.github.com>

Author: hyperpolymath

lib/codegen_deno.ml

@@ -191,7 +191,8 @@ const __as_walkRecursive = (root) => {
 };
 const __as_regexMatch = (s, pat) => new RegExp(pat).test(String(s));
 const __as_wasmInstance = (bytes) =>
-  new WebAssembly.Instance(new WebAssembly.Module(bytes)).exports;
+  new WebAssembly.Instance(new WebAssembly.Module(bytes),
+    { wasi_snapshot_preview1: { fd_write: () => 0 } }).exports;
 const __as_wasmCall = (exports, name, args) => Number(exports[name](...(args || [])));
 // ---- WasmValue (Deno.affine #455 — Tier 1 #5, Option B) ----
 // Opaque tagged value crossing the AS/JS boundary as `{ kind, v }`.

tests/codegen-deno/wasm_wasi_instance.affine

@@ -0,0 +1,16 @@
+// SPDX-License-Identifier: MPL-2.0
+// Regression for the Deno-ESM WASI-stub fix: `__as_wasmInstance`
+// (lib/codegen_deno.ml) must be able to instantiate a wasm that imports
+// `wasi_snapshot_preview1.fd_write` — which EVERY affinescript-compiled wasm
+// does, unconditionally — and not only an import-free module. Before the fix
+// the shim instantiated with no import object and threw "Imports argument must
+// be present and must be an object", so the deno-esm backend could not load any
+// affinescript-produced wasm. Companion to wasm_call.affine (closes #414); this
+// fixture exercises the wasmInstance lowering against a WASI-importing module.
+
+use Deno::{ Bytes, WasmExports, wasmInstance, wasmCall };
+
+pub fn addViaWasiWasm(bytes: Bytes, a: Float, b: Float) -> Float {
+  let exports = wasmInstance(bytes);
+  wasmCall(exports, "add", [a, b])
+}

tests/codegen-deno/wasm_wasi_instance.harness.mjs

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: MPL-2.0
+// Node-ESM harness for the WASI-stub regression (wasm_wasi_instance.affine).
+//
+// Instantiates an 86-byte wasm module that IMPORTS
+// `wasi_snapshot_preview1.fd_write` (declared, never called) and exports
+// `add(i32,i32)->i32` — the same import shape every affinescript-compiled wasm
+// carries. `addViaWasiWasm` lowers to `__as_wasmInstance(bytes)` followed by
+// `__as_wasmCall(...)`; the call succeeds only when `__as_wasmInstance` supplies
+// the `{ wasi_snapshot_preview1: { fd_write: () => 0 } }` import object. Before
+// the fix this threw "Imports argument must be present and must be an object".
+
+import assert from "node:assert/strict";
+import { addViaWasiWasm } from "./wasm_wasi_instance.deno.js";
+
+// (module
+//   (import "wasi_snapshot_preview1" "fd_write"
+//     (func (param i32 i32 i32 i32) (result i32)))   ;; imported, never called
+//   (func (export "add") (param i32 i32) (result i32)
+//     local.get 0  local.get 1  i32.add))
+const wasmBytes = new Uint8Array([
+  0x00, 0x61, 0x73, 0x6d, 0x01, 0x00, 0x00, 0x00,                   // magic + version
+  0x01, 0x0f, 0x02,                                                 // type section: 2 types
+  0x60, 0x04, 0x7f, 0x7f, 0x7f, 0x7f, 0x01, 0x7f,                   //   (i32,i32,i32,i32)->i32  (fd_write)
+  0x60, 0x02, 0x7f, 0x7f, 0x01, 0x7f,                               //   (i32,i32)->i32          (add)
+  0x02, 0x23, 0x01,                                                 // import section: 1 import
+  0x16, 0x77, 0x61, 0x73, 0x69, 0x5f, 0x73, 0x6e, 0x61, 0x70,       //   "wasi_snap...
+  0x73, 0x68, 0x6f, 0x74, 0x5f, 0x70, 0x72, 0x65, 0x76, 0x69,       //   ...shot_previ...
+  0x65, 0x77, 0x31,                                                 //   ...ew1"
+  0x08, 0x66, 0x64, 0x5f, 0x77, 0x72, 0x69, 0x74, 0x65,             //   "fd_write"
+  0x00, 0x00,                                                       //   kind=func, type 0
+  0x03, 0x02, 0x01, 0x01,                                           // function section: func 1 of type 1
+  0x07, 0x07, 0x01, 0x03, 0x61, 0x64, 0x64, 0x00, 0x01,             // export "add" -> func index 1
+  0x0a, 0x09, 0x01, 0x07, 0x00, 0x20, 0x00, 0x20, 0x01, 0x6a, 0x0b, // code: local.get 0/1, i32.add
+]);
+
+assert.equal(addViaWasiWasm(wasmBytes, 2, 3), 5, "addViaWasiWasm(2,3) == 5 (WASI-importing module instantiates)");
+assert.equal(addViaWasiWasm(wasmBytes, -1, 1), 0, "addViaWasiWasm(-1,1) == 0");
+assert.equal(addViaWasiWasm(wasmBytes, 100, 200), 300, "addViaWasiWasm(100,200) == 300");
+
+console.log("wasm_wasi_instance.harness.mjs OK");