compat gate: consolidate into one required status check

[rubenhensen]

Sub-issue of #251 (epic #247, workstream C). Make it unmergeable.

• Consolidate the Rust + JS halves under one named required check (wire-compat).
• Document the gate and the readers-first rollout rule in CONTRIBUTING.md (a red gate means: make the change genuinely additive, or ship read support a release ahead of the write flip).
• Branch protection (requiring the check on main PRs) is an admin action — flagged for the maintainer once the check exists.

Done when: wire-compat appears as a single status on PRs and branch protection lists it as required.

[rubenhensen]

PR up: #295 adds a wire-compat job to build.yml (needs: [wire-compat-rust, wire-compat-js], reports one pass/fail) and documents the gate + readers-first rollout rule in a new CONTRIBUTING.md.

Not closing yet — this map's done bar is merged-to-main-with-CI-green, and two things are still open:

1. CI on ci: consolidate wire-compat into one required status check #295 is still running.
2. Branch protection still requires the two old per-language checks (Wire compat (published pg-core), Wire compat (published pg-wasm/pg-js)); repointing it at wire-compat is an admin action, called out in the PR body and left for a maintainer per this ticket's own text.

Will update once #295 is green and merged.