Adds a Config.Walker callback that fires on rule-content edit /
partition flip BEFORE the cursor rewinds, so already-due objects across
the rewritten rule set get caught instead of waiting on meta-log
replay alone. The callback receives engine.RecoveryView(snap) and the
per-shard ID; nil disables it (Phase 4a behavior preserved).
Decoupling the wiring from the implementation: the handler-side
WalkerFunc that drives bootstrap.Walk via the filer is the follow-up
commit, and tests can stub the callback without standing up the full
filer/client/lister harness.
Tests pin: walker fires exactly once on hash mismatch, walker error
propagates and leaves the cursor unchanged, nil Walker is a no-op.
Chris Lu
d2b9f53edb