tracking: single-process root-set SST GC

[belveryin]

Summary

Track the single-process root-set SST GC work.

This is intentionally scoped to the single-process Tonbo model. It is not a distributed GC plan, and it should not be read as the first incremental step toward distributed GC. When Tonbo needs distributed GC, that should be specified in a separate RFC with a different mental model.

Retention policy is also intentionally out of scope for this first milestone. The current GC work should not introduce max_ttl, max_versions, or broader time-travel semantics yet.

Child Issues

• bench: establish SST GC baseline and observability harness before enablement #594
• refactor: make the current root set the only SST GC authority #595
• feat: implement current-root-set builder for GC #596
• feat: add SST sweeper that deletes only unreachable manifest-published objects #597
• test: add single-process GC correctness coverage for root-set protection #598
• feat: add minimal GC metrics for storage savings and latency cost #599
• chore: document WAL GC as a standalone floor-driven subsystem #600

Notes

• Baseline-first benchmarking and observability are part of the definition of done for each item.
• WAL GC stays standalone and floor-driven in this effort.
• Benchmarks should follow the adjacent-repo pattern: explicit baselines like aisle, benchmark-integrated counters like fusio-manifest.
• Retention policy should be added later as a separate follow-up issue, not folded into the first GC delivery.

[linear]

DEV-526