fix(snif): make Wasmex an optional, absence-guarded dependency (#46)

## Why

`Burble.Coprocessor.SNIFBackend` referenced `Wasmex.start_link/1` and
`Wasmex.call_function/3` directly while `:wasmex` is not even a declared
dependency, and `available?/0` only checked the `.wasm` file existed.
That produced a compile-time `Wasmex … is undefined` warning and the
wrong runtime failure mode (rescued exception per call instead of clean
degradation).

## What

Applies the accepted `:quicer` / `Burble.Bolt.Quic` pattern from
**ADR-0004** to `:wasmex`:

* **`server/mix.exs`** — `:wasmex` documented in the existing
optional-NIF
  block (Rust/wasmtime toolchain), commented like `quicer`/`elmdb`.
* **`snif_backend.ex`** — `@wasmex Wasmex` alias; calls via `apply/3` so
  the compiler does not warn when absent; `available?/0` now also
  requires `Code.ensure_loaded?(@wasmex)` + `function_exported?`, so the
  existing `if available?()` guards transparently fall back to
  `ZigBackend`.
* **CHANGELOG** — Fixed entry.

## Verification

`mix compile --force --no-deps-check` → no `Wasmex` warning, no new
unused/undefined warnings in `snif_backend.ex`. Pure consistency/bugfix;
no behaviour change when Wasmex *is* present.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

CHANGELOG.md

@@ -21,6 +21,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - `prim__registerCallback` made module-private (unsafe boundary, awaits idris2#3182)
 - `docs/developer/wsl-mirrored-networking.adoc` rewritten — NAT + host forwarder is the recommended WSL2 Bolt path; mirrored networking demoted to last-resort (Win11 24H2/Insider `Wsl/Service/E_UNEXPECTED` instability)
 
+### Fixed
+- SNIF: `Burble.Coprocessor.SNIFBackend` no longer emits a compile warning for the optional `Wasmex` runtime and no longer mis-fails when it is absent — `Wasmex` is referenced via `apply/3` and `available?/0` now gates on it loadable, so kernels degrade cleanly to `ZigBackend` (mirrors the `:quicer` pattern, ADR-0004)
+
 ### Removed
 - TODO.md (superseded by CLAUDE-WORK.md — 0 TODOs remain in codebase)
 

server/lib/burble/coprocessor/snif_backend.ex

@@ -157,9 +157,17 @@ defmodule Burble.Coprocessor.SNIFBackend do
   3. Verify configuration in `config/runtime.exs`
   4. Check `BURBLE_SNIF_PATH` environment variable
   """
+  # Optional WASM runtime. Referenced via apply/3 (see call_snif_module/3)
+  # so the compiler does not warn when :wasmex is absent at build time —
+  # mirrors the Burble.Bolt.Quic / :quicer pattern (ADR-0004). When absent,
+  # available?/0 is false so every kernel transparently uses ZigBackend.
+  @wasmex Wasmex
+
   @impl true
   def available? do
-    File.exists?(@snif_path)
+    File.exists?(@snif_path) and
+      Code.ensure_loaded?(@wasmex) and
+      function_exported?(@wasmex, :start_link, 1)
   end
 
   # ---------------------------------------------------------------------------
@@ -512,9 +520,9 @@ defmodule Burble.Coprocessor.SNIFBackend do
   # `{:error, reason}` so the caller can fall back gracefully.
   defp call_snif_module(path, function, args) do
     try do
-      case Wasmex.start_link(%{bytes: File.read!(path)}) do
+      case apply(@wasmex, :start_link, [%{bytes: File.read!(path)}]) do
         {:ok, pid} ->
-          result = Wasmex.call_function(pid, function, args)
+          result = apply(@wasmex, :call_function, [pid, function, args])
           GenServer.stop(pid, :normal)
           result
         {:error, reason} -> {:error, {:wasm_load_failed, reason}}

server/mix.exs

@@ -196,10 +196,14 @@ defmodule Burble.MixProject do
       #   quicer  — needs msquic (Microsoft QUIC C library)
       #   elmdb   — links liberl_interface which was dropped in OTP 23+
       #   ex_lmdb — depends on elmdb
+      #   wasmex  — Rust NIF (wasmtime); needs a Rust toolchain. SNIF
+      #             (Burble.Coprocessor.SNIFBackend) transparently degrades
+      #             to ZigBackend when absent — available?/0 gates on it.
       #
       # {:quicer, github: "emqx/quic", tag: "0.2.15", submodules: true, optional: true},
       # {:elmdb, "~> 0.4", optional: true},
       # {:ex_lmdb, "~> 0.1", optional: true},
+      # {:wasmex, "~> 0.9", optional: true},
 
       # Media plane — ex_webrtc SFU (audio-only, Opus)
       {:ex_webrtc, "~> 0.16"},