[#446 Tier 1 #5 follow-up] Zig FFI implementation for wasm_export_call dispatch #497

Context

Closed #455 (the WASM-exports calling pattern kickoff) shipped via PR #467 with the Deno-ESM backend lowering: wasm_export_call(exports, name, args) + WasmValue opaque type + JS-side dispatch (__as_wasm_export_call).

Per the owner's Option-B scope breakdown, the Zig-side dispatch was deferred to a follow-up:

Zig FFI: implement the call dispatch (marshal args, find export by name, call, marshal return)

This issue tracks that.

Scope

The Zig-side counterpart to the Deno-ESM lowering. When wasm_export_call is invoked from an AffineScript program targeting the linear-memory WebAssembly backend (i.e. anywhere outside the Deno-ESM host), the host must supply a wasm_export_call symbol under the env import — and that symbol should be implementable in a canonical Zig crate so any embedder can pick it up.

What to ship

• A canonical Zig crate (in this repo's tools/, or scaffolded as a hpm-wasm-export-rsr sibling — owner's choice) exporting an extern "C" fn wasm_export_call(exports_handle: *anyopaque, name_ptr: [*]const u8, name_len: usize, args_ptr: [*]const WasmValueRepr, args_len: usize) -> WasmValueRepr (or matching ABI).
• Marshalling: WasmValueRepr ↔ the tagged scalar (i32 / i64 / f32 / f64).
• Export lookup: by name through a WebAssembly.Instance.exports-like handle.
• Recipe doc (or row in the existing docs/specs/zig-ffi-patterns.adoc) showing how to wire this Zig dispatcher into a wasm-host configuration.

Acceptance

• Zig source compiles to a freestanding .wasm (or a host-linkable .so) that satisfies the env.wasm_export_call import contract.
• Round-trip exercised by a smoke-test (see [#446 Tier 1 #5] WASM-exports calling pattern (recommended kickoff) #455 follow-up "Zig-FFI smoke-test" sub-issue).
• STDLIB-EXTERN-AUDIT.adoc row for wasm_export_call updated to flag the Zig host as available.

Refs

• Closed kickoff: [#446 Tier 1 #5] WASM-exports calling pattern (recommended kickoff) #455
• Shipped surface: PR feat(stdlib): WasmValue + wasm_export_call typed wasm-exports binding (closes #455) #467 (Deno-ESM lowering + Deno.affine externs)
• Companion doc: PR Roadmap tracking + Zig-FFI doc (#19) + hex/base64 encoding (#25) + Int-division codegen fix (#478) #474 — docs/specs/zig-ffi-patterns.adoc (general pattern doc)
• Umbrella: [campaign] AffineScript top-50 framework bindings roadmap (UMBRELLA) #446 (Tier 1 — idaptik blockers)
• Tier sub-issue: [#446 STEP 2] Tier 1 — idaptik blockers (10 bindings) #450
• Estate directive: "Zig = APIs + FFIs"

This is the FFI implementation half; the Idris2-ABI proof-carrier doc is a separate follow-up under the "Idris2 = ABIs" directive.