ci: speed up test suite, especially SEA tests #241

The CI matrix currently runs three parallel suites (test:host, test:22, test:24) across Ubuntu, Windows, and macOS. SEA tests in particular are slow because each test invocation downloads or rebuilds a Node.js binary and runs the full SEA pipeline (blob generation, postject injection, signing).

Areas to investigate

• Caching — Cache downloaded Node.js binaries and pkg-fetch artifacts across CI runs
• Test deduplication — Identify tests that overlap between host/22/24 suites and can be skipped for certain targets
• Parallelism — Run independent test directories concurrently within each suite
• SEA-specific — Pre-build the SEA blob or share intermediate artifacts across test cases to avoid redundant work

Goal

Significantly reduce wall-clock CI time without sacrificing coverage.

Part of #235