feat: implement current-root-set builder for GC

[belveryin]

Why

We need a deterministic protected-object set derived from HEAD plus any existing hard pins, without pulling retention policy into the first GC milestone.

Scope

• Enumerate HEAD and any existing hard pins from manifest state.
• Resolve SST data and delete-sidecar paths for those versions.
• Return a protected object set for a table.

Acceptance Criteria

• Protected set includes HEAD and any existing hard pins.
• Unit tests cover root-set edge cases.
• Metrics expose root-set version count and protected object count.

Explicit Non-Goals

• Do not introduce max_ttl.
• Do not introduce max_versions.
• Do not add time-travel retention semantics in this issue.

[linear]

DEV-529