Track Zig 0.17 upstream dependency migration and remove vendored snapshots

[justrach]

Goal\n\nRemove the temporary vendor/dhi and vendor/turboapi-core snapshots once upstream Zig 0.17-compatible revisions are available.\n\n## Upstream issues\n\n- dhi: https://github.com/justrach/dhi/issues/69\n- turboapi-core: https://github.com/justrach/turboapi-core/issues/3\n\n## Completion checklist\n\n- [ ] Both upstream projects compile and test under the supported Zig 0.17 toolchain.\n- [ ] Replace the path dependencies in build.zig.zon with upstream URLs/hashes.\n- [ ] Remove the corresponding vendor directories.\n- [ ] Run zig fmt --check, zig build, zig build test, and the benchmark suite.\n- [ ] Update migration.md and close this issue.\n\nSee migration.md for the current compatibility notes.