Report unresolved browser WebAssembly host imports through wavec run

## Problem

Running a browser-oriented WebAssembly module through `wavec run` can expose a raw Node.js instantiation error when the module imports host functions.

For example, `examples/wasm_module.wave` imports `env.host_add`. Running it for `wasm32-unknown-unknown` currently ends with a message similar to:

```text
LinkError: WebAssembly.instantiate(): Import #0 module="env" function="host_add": function import requires a callable
```

The built-in runner cannot invent application-specific browser host functions, but it should explain the contract cleanly.

## Scope

- Inspect WebAssembly imports before instantiation, or normalize the runtime instantiation failure.
- Report the missing module/function names through a stable `wavec` diagnostic.
- Explain that modules with custom browser host imports require an explicit JavaScript host runner.
- Preserve successful `wavec run` behavior for modules that need no custom imports.
- Add a regression based on the existing browser-hosted example.

## Acceptance criteria

- No raw Node.js stack or unclassified `LinkError` is shown for missing host imports.
- The diagnostic names `env.host_add` and provides an actionable next step.
- Standalone WebAssembly execution remains covered.

Start with the `wavec run` implementation and reproduce the failure using `examples/wasm_module.wave` for `wasm32-unknown-unknown`. Inspect where WebAssembly imports are instantiated, add a regression for the browser-hosted example, and verify that missing `env.host_add` produces a stable actionable diagnostic while modules without custom imports still run.

Written by the indexing model from the issue text.