ci: run independent checks in parallel

[dqn]

Summary

CI now runs independent GitHub Actions checks in parallel where the jobs already share the same setup.

Verification

PR checks completed with 0 failures, and GitHub Actions accepted step-level parallel syntax in ci and Template Tests.

Area	Sequential step total	Parallel wall time	Observed effect
ci lint/format group	~21s	11s	~10s saved
ci post-generate checks	~30s	9s	~21s saved
Template tests (Ubuntu)	~23s	14s	~9s saved
Template tests (Windows)	~45s	28s	~17s saved

Notes

Whole-job timings are noisy because install and individual command duration varied between runs. zizmor v1.26.1 still warns on workflows that use parallel, so this PR should stay draft until we decide whether that tooling coverage gap is acceptable or wait for upstream support.

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Latest commit: 699555c

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