ci: add build-wasm job to verify WebAssembly compilation

[Mossaka]

Summary

• Adds a new build-wasm job to the CI pipeline that verifies the WebAssembly compilation target (make build-wasm) succeeds
• Reports the resulting .wasm binary size to GITHUB_STEP_SUMMARY for visibility
• Follows the same patterns as existing CI jobs (pinned action SHAs, retry logic for module downloads, concurrency groups)

Test plan

• Verify the build-wasm job appears in the CI workflow run
• Confirm make build-wasm completes successfully in CI
• Check that the binary size is reported in the GitHub Actions step summary

🤖 Generated with Claude Code

[Copilot]

Pull request overview

Adds CI coverage for the project’s WebAssembly build target so CI will fail if make build-wasm stops compiling, and surfaces the produced .wasm size in the run summary.

Changes:

• Introduces a new build-wasm CI job that runs make build-wasm on Ubuntu.
• Adds Go cache reporting and Go module download/verify steps consistent with other Go jobs.
• Appends the resulting gh-aw.wasm size to GITHUB_STEP_SUMMARY.

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[Copilot]

In the failure branch, ls -la *.wasm writes to the job logs, not to $GITHUB_STEP_SUMMARY, so the step summary won’t include the directory listing when a .wasm exists but gh-aw.wasm is missing. Consider redirecting the listing output into the step summary as well (and using a file test like [ -f gh-aw.wasm ] instead of ls for existence checks).