feat(bucket-object): add object entry state model [10/n]

[sadhansood]

Description

This adds the next bucket_object contract slice: the per-key object state model. The new modules introduce:

• ObjectVersion as the immutable version record for an object update
• ObjectEntry as the shared per-key object state
• current_version / pending_version tracking
• generation-based staging and promotion rules for pending versions

This PR is still intentionally contract-only and keeps the scope narrow. It does not wire blob registration, certification, or object update APIs yet. The goal here is to land the core object state shape and its invariants before building the write flow on top of it.

Test plan

• rustup run 1.86 cargo fmt -- --config group_imports=StdExternalCrate,imports_granularity=Crate,imports_layout=HorizontalVertical
• cargo clippy --workspace --all-features --tests --no-deps -- -D warnings
• sui move test --path /Users/sadhansood/dev/walrus/contracts/bucket_object

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[github-actions]

Warning: This PR modifies the Walrus CLI. Please consider the following:

• Make sure the changes are backwards compatible. Consider deprecating options before
  removing them.
• Generally only use long CLI options, not short ones to avoid conflicts in the
  future.
• If you added new options or features, or modified the behavior, please document the
  changes in the release notes of the PR and update the documentation in the
  docs/content directory.