[Epic] Compaction & space reclamation semantics

When Harper reclaims disk space, when it doesn't, and whether the difference is intentional.

## Scope

**In:** compaction behavior and its accounting, tombstone reclamation after bulk delete or TTL churn, transaction-log retention, and which stores are eligible for compaction at all.

**Out:** the storage-reclamation *trigger* plumbing that decides when to run (largely rewired by #650), except where retention policy is the thing in question.

## Why

The open questions here are semantic before they're mechanical: whether space is reclaimed or merely reused is unresolved (#1384), whether transaction logs need a retention control independent of pressure signals is unresolved (#846), and #596 suggests the accounting we'd use to answer either may itself be wrong. Grouping them so the semantics get decided once, rather than three times in three different fixes.

_Created during backlog triage. Child issues keep their own priorities and milestones._

Start by reviewing the child issues referenced in the epic, especially #1384, #846, and #596, along with the compaction and storage code they identify. Done means the reclamation, transaction-log retention, accounting, and store-eligibility semantics are resolved consistently; no specific file or test is named here.

Written by the indexing model from the issue text.