ci: add fmt/build checks and keep test/race workflow

[cpunion]

Summary

This PR improves GitHub Actions CI coverage for github.com/goplus/plan9asm by adding dedicated checks and keeping the existing test coverage.

Added jobs

• fmt: verifies formatting with gofmt -l .
• build: verifies package build with go build ./...
• race: runs go test -race ./... on Go 1.22.x

Existing test coverage retained

• test matrix on Go 1.21.x and 1.22.x
• go test ./... for each matrix entry

Workflow hardening / performance

• explicit minimal workflow permission:
  • permissions: contents: read
• actions/setup-go caching enabled
• fail-fast: false kept for matrix visibility when one version fails

Why

• Catch formatting and compile regressions earlier
• Keep runtime race detection as a separate signal
• Maintain compatibility checks across supported Go versions

Verification (local)

gofmt -l .
go build ./...
go test ./... -count=1
go test -race ./... -count=1

[gemini-code-assist]

Note

Gemini is unable to generate a summary for this pull request due to the file types involved not being currently supported.