Testing: PR gate restructuring and main.yml (T-05) #339

Part of the test strategy (captain, 2026-10-05: scenarios that test the changes together; cleaner, faster suites; TESTING.md). Epic #305.

Scope. ci.yml: Tests job runs cargo test --workspace --all-features (now including scenarios), then scripts/run_snapshot_tests.sh --debug --affected <merge-base>, then prints the per-binary timing table; new JS SDK job (path filter adds ws-protocol/**, docs/spec/**, src/protocol/** to both SDK jobs); new .github/workflows/main.yml on push to main: full spec corpus, oracle 50 seeds, js-test-live, python-sdk-live, python-test-live, release e2e-reactive. run_snapshot_tests.sh --debug uses target/debug binaries.

Acceptance. A PR touching src/ finishes Tests in under 10 min wall on blacksmith-4vcpu; the three #290 specs fail the first main.yml run (then are fixed by #290); a PR touching only packages/inputlayer-js runs the JS job and nothing Rust.