bindings #5: WASM-exports calling pattern #414

Child issue of #411 (bindings umbrella). Item bindings #5 in docs/bindings-roadmap.adoc.

What

Provide a way to invoke individual exports.fn_name(args) from a WasmExports value, from AffineScript code.

stdlib/Deno.affine already exposes:

• WasmExports type
• wasmInstance(bytes) -> WasmExports
• readFile

What's missing is the calling surface — given a WasmExports value, how do you actually call individual exports by name?

Why

• Idaptik vm/wasm (740 LoC across 2 files) is blocked here today.
• Every future WASM-host integration in the estate will need this surface.
• Smallest scope / highest leverage Tier-1 item in the bindings roadmap.

Two approaches (pick one)

Option A — Generic dynamic call: one extern, ~10 lines:

extern fn wasm_call(exports: WasmExports, name: String, args: [Float]) -> Float

Pros: tiny surface, future-proof. Cons: stringly-typed, no compile-time check that the export exists.

Option B — Per-export typed externs: ~30 extern fn declarations matching the specific exports idaptik's vm/wasm Zig module ships.
Pros: typed, compile-time-checked. Cons: brittle to changes in the Zig module's export list.

The roadmap doc leans toward Option A as the generic surface, with Option B's typed shape achievable via per-consumer wrapper modules layered on top.

Where it lives

stdlib/Deno.affine extension, or a new stdlib/wasm.affine (decide as part of this issue).

Done when

• AS-side surface lands (extern fn(s) declared)
• Host-side glue lands (Deno side)
• At least one consumer demonstrates the round-trip (idaptik vm/wasm would be the natural one)
• Documented in docs/bindings-roadmap.adoc (status row updated ◐ → ●)
• Idaptik PR Cross-module imports: TopConst not threaded by gen_imports / flatten_imports #107 successor unblocked on this dimension

Refs

• docs/bindings-roadmap.adoc — bindings ci: Bump actions/github-script from 7.0.1 to 8.0.0 #5
• Idaptik PR Cross-module imports: TopConst not threaded by gen_imports / flatten_imports #107 (the pilot that surfaced this need)
• stdlib/Deno.affine — current WasmExports surface