Adds internal/storage/faultstore, a storage.Storer wrapper that injects faults through the storage seam without patching production code: an upload that dies mid-stream, a backend reporting success while storing nothing, and reads returning corrupt or truncated bytes. Covers all six scenarios from the issue, each asserting the observable end state (index, destination, and what the user is told), not merely that an error returned. Scenario 1b (retry after an interrupted upload) exposed a real dedup defect and is skipped with a pointer to #148, which also owns the half-exported-state repair. Tests run serially because each calls log.Initialize on the global logger. No production behavior changes.
Model: opus-4-8
