chore: document WAL GC as a standalone floor-driven subsystem

[belveryin]

Why

The SST GC work should not destabilize the current WAL GC path.

Scope

• Make the separation explicit in code comments and docs.
• Avoid introducing shared machinery unless there is a concrete need.
• Call out that retention policy for SST GC is deferred and not part of the current WAL work.

Acceptance Criteria

• WAL GC remains floor-driven.
• SST GC changes do not expand WAL GC scope.

[linear]

DEV-533