Add host-IB zero-copy transport + per-peer mapper cache

[Regina8023]

Summary:
Introduce the host-side IB P2P transport stack, end-to-end for the
pure zero-copy (ZC) mode. This is the first of two diffs; the
copy-based (CB) mode lands in a follow-up.

What's in this diff:

• ctran::transport::IP2pHostTransport — backend-neutral abstract
  interface algorithms see. Carries pure ctrl-msg primitives
  (iSendCtrl/iRecvCtrl take raw void* payloads + size) and
  mode-agnostic SendChunkArgs / RecvChunkArgs.
• ctran::transport::ib::HostZcTransport — pure-ZC concrete impl. No
  staging buffers, no state machine. iSendChunk is a thin wrapper
  on vcs_[vcIdx]->iput; iRecvChunk bumps per-VC notify counters.
• ctran::transport::ib::impl — shared ctrl-plane helpers
  (ensureVcsReady, iSendCtrlImpl, iRecvCtrlImpl, testCtrlDoneImpl,
  waitCtrlDoneImpl, progressImpl, exportRecvBuf, importRemoteInfo,
  toIbRegElem, CtrlRequestPriv friend struct).
• CtranMapper::getP2pTransport(peer, mode) per-peer cache. The
  kCopyBased branch is a temporary FB_CHECKABORT pointing at the
  follow-up diff.
• MockP2pHostTransport.h for unit-testing algos against the
  interface.
• ctran_mapper_host_transport_ut: cache lifecycle + per-peer
  caching tests (ZC scope).
• host_transport_dist_ut: end-to-end ZC RoundTrip +
  BidirectionalRoundTrip across {1, 2, 4} channels on real IB.
• HostTransportDesign.md: full architecture doc describing both ZC
  and CB. CB sections describe the API + state machine the
  follow-up diff implements.

Reviewed By: minsii

Differential Revision: D106138307

[meta-codesync]

@Regina8023 has exported this pull request. If you are a Meta employee, you can view the originating Diff in D106138307.