feat(aot): `--compile --target=` should cross-compile against Home runtime artifacts, not Bun's npm binaries

Problem

The ported CompileTarget downloads @oven/bun-<os>-<arch> from registry.npmjs.org, so a "cross-compiled Home executable" is really upstream Bun with a Home-built bundle inside.

Evidence (origin/main @ d4b219507)

• packages/runtime/src/options_types/CompileTarget.zig:3-5,75-95,164-195
• packages/runtime/src/standalone_graph/StandaloneModuleGraph.zig:1041-1080

Scope

• Publish per-target Home runtime templates on GitHub Releases with checksums.
• Point CompileTarget at them and verify the checksum before embedding.

Acceptance criteria

• For each supported target, the embedded runtime reports Home's version, not Bun's.
• A checksum mismatch fails the build.

Links

Depends on: #791 Part of #814.