Add the shared design/contract document for backlog #1326: a single
per-object generation authority (the #1312 fencing epoch) that spans
commit fencing, read lease, prepared pool read, quota reservation, and
old-dir GC. Pins the five cross-cutting constraints once - RPC signature
binding with server-side nonce enforcement, xl.meta encoding contract
(no meta_ver bump, no positional FileInfo field, internal metadata map
under the dual-key contract), proto3 optional presence, mixed-version
fallback direction, and cluster-level capability negotiation - so the
implementation sub-issues follow them rather than each re-deciding.
No product code changes.
Zhengchao An