feat(aot): ship the JS/TS runtime in Linux release binaries

Problem

Servers run Linux, but JSC is only enabled by default on macOS and the release workflow never turns it on, so home run / home build --compile on .ts do not work in Linux releases. Linking also depends on a local Bun checkout at ~/Code/bun/build/release/obj, falling back to the system framework on macOS only.

Evidence (origin/main @ d4b219507)

• build.zig:408
• build.zig:189-220
• .github/workflows/release.yml:76
• src/main.zig:3497-3503

Scope

• Enable the JS runtime for x86_64-linux-gnu and aarch64-linux-gnu release builds.
• Fetch WebKit/Bun build artifacts hermetically (pinned, checksummed) instead of from a local checkout.

Acceptance criteria

• The Linux release tarball runs home build --compile server.ts and the result serves HTTP in a clean container.
• The release job has no dependency on a developer-machine path.

Links

Related: #66, #753 Part of #814.