ci: speed up GitHub Actions

[jayzeng]

Summary

• run feature-branch CI only through pull_request, while retaining push verification for main
• cancel superseded workflow runs and consolidate the fast path into one Ubuntu verification job plus one Windows unit job
• move the Windows qmd smoke into a path-filtered workflow with an isolated, versioned qmd cache
• move API-backed e2e to an explicit manual workflow and run the correct npm run test:e2e command
• add unit regression coverage for the intended workflow topology

Recent paired CI runs consumed about 12 runner-minutes per commit (335 seconds for pull_request plus 385 seconds for the duplicate push run). The Windows qmd smoke was the critical path at 119-178 seconds, mostly from installing qmd.

Test plan

• Baseline: npm test — 168 passed
• Red: npm test — failed as expected because the new split workflow files did not exist
• Green: npm test — 172 passed
• npm run build
• npm run lint
• npm pack --dry-run
• npm audit --omit=dev — 0 vulnerabilities
• ruby -e 'require "yaml"; Dir[".github/workflows/*.yml"].sort.each { |f| YAML.parse_file(f) }'
• go run github.com/rhysd/actionlint/cmd/actionlint@v1.7.12
• git diff --check

Compatibility

• No on-disk memory format changes.
• No runtime qmd behavior changes; only its Windows CI smoke-test scheduling and installation cache changed.