test: pin the review boundaries for pins, promotion, diff and cleanup (RFC 0067)

Seven `failpoints.rs` cases: the single-link pending-pin diff control, the
blocked-chain and pending-merge diffs, cleanup on a blocked pin, the aged
detached-manifest reaper, the reader racing a promotion, and a stale
handle writing after another handle's promotion. The five that reproduced
the review's defects failed before the fixes. The maintenance and write
guides describe the blocked-pin GC refusal and the age-gated reaper, and
the blocked-pin sentence now says branch merge and index maintenance stage
behind a block.

Author: ragnorc