chore: migrate to Zig 0.17.0

[vmvarela]

Summary

Migrate oci.zig from Zig 0.16.0 to Zig 0.17.0 with the smallest compatibility surface required by the compiler and tests.

Changes

• bump minimum_zig_version and all CI Zig pins to 0.17.0
• replace removed array repetition syntax (**) with @splat
• migrate enum reflection from .fields to Zig 0.17 .field_names
• migrate URI host extraction from removed Uri.getHost / getHostAlloc APIs to std.Io.net.HostName.fromUri
• update current Zig-version references in README, SPEC, and source comments
• keep historical references to std.Io being introduced in Zig 0.16 where they remain accurate

Validation

CI is green on Zig 0.17.0 for:

• unit tests on Linux
• unit tests on macOS
• unit tests on Windows
• integration tests against zot
• public-registry smoke tests against Docker Hub

No dependency changes and no unrelated refactors.

Closes #15