Add self-contained Git bundle creation - #7434

PR description:

Follow-up to #7338. Add git_bundle_create to write self-contained bundles from explicitly selected references, completing the basic offline create-and-clone workflow.

• Write v2 bundles for SHA-1 repositories and v3 bundles for SHA-256 repositories.
• Include complete reachable history, preserving annotated tags and tag chains, including tag-only selections.
• Advertise references in caller order, deduplicate repeated selections, and include HEAD only when explicitly selected.
• Stream packbuilder output through atomic file replacement, with progress callbacks, cancellation, and cleanup on failure.
• Reject shallow sources, invalid selections, and missing reachable objects.

Incremental creation, filtered bundles, and bundle-URI support remain out of scope.

This continues the bundle work originally proposed by Laurence McGlashan in #7101 and explored in #7221.

Validation

• Warnings-as-errors build passes.
• Bundle creation, parser, clone, fetch, transport-selection, and struct initialization tests pass.
• Git interoperability checks pass for SHA-1 and SHA-256 bundles: git bundle verify, bare clone, git fsck --full, and complete parent history for branch and tag-only selections.
• git diff --check passes.

Sanitizer and Windows validation remain outstanding.