ci: parallelize the full native release gate

[pathscale]

The native release gate took more than seven minutes because all 233 theme-editor outcomes ran serially in one browser, after each runner rebuilt Chuzz from source. This keeps every one of the 675 release checks and partitions the theme corpus at clean browser-state boundaries so six jobs can run the complete gate in parallel. The build is produced once and shared with each native runner.

The README now makes the release rule explicit: all 675 checks are mandatory, the matrix is not a smoke suite, and ps-qa 0.7.3 is the required floor. The workflow matrix contains all 18 groups exactly once.

Local validation on the production build:

• bun run typecheck
• bun run build
• bun run qa:production-policy
• full native ps-qa release gate: 675/675 across 18 groups
• strict native inventory: 353/353 outcome-declared; zero missing IDs, unstable IDs, duplicate IDs, or unverified controls
• six isolated theming partitions: 233/233
• YAML parse and matrix/group reconciliation