Add WASM integration tests for Node

[ikskuh]

Summary

• export the HyperDoc WASM API symbols during the build so JavaScript consumers can call into the module
• flush WASM writers so HTML output and diagnostics are captured
• add a Node-based integration script and fixtures that validate HTML snapshots and diagnostics from the WASM build
• wire the node-based WASM integration tests into zig build test when Node is available, and emit a warning when it is not
• document the optional Node.js dependency for running the WASM tests

Testing

• zig build
• zig build test
• node test/wasm/validate.js

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Codex Task