[Spicy-cream]

What problem does this PR solve?

URMA can fall back to TCP during connection setup, but established failures do not control subsequent admission. A failed server SEND can close the control socket while the client immediately reconnects over URMA to the same peer.

What is changed and the side effects?

• Add process-level LEGACY, AUTO and TCP_ONLY modes. AUTO applies a configurable peer cooldown, defaulting to 5 seconds; LEGACY remains the default.
• Report completion, JFC and WR posting failures once per connection, filter earlier epochs, count access errors separately and preserve EAGAIN for backpressure.
• Report established client control-socket EOF/read failures so server-side faults also trigger peer cooldown. Normal server-side EOF from short clients closes the connection without quarantining the peer.
• Select CTP/RTP paths from device capabilities, release short-connection resources, serialize TCP reads and initialize/validate send SGEs. Optional aligned copied sends add one payload copy.
• Add a recovery client, process-local SEND injection, independent regressions and a URMA mock workflow for pull requests, including non-master targets.

Mode changes affect new connections. Short connections can resume URMA after cooldown; established single/pooled TCP connections can continue serving RPCs. Existing v2/v3 wire formats remain compatible.

Validation

• Synchronized with Apache master at 2a3a186. The CI configure/build/test commands pass locally with RDMA disabled and URMA enabled: 15 regression groups passed.
• The server-SEND regression fails before the control-socket fix. All 8 CTP/RTP, v2/v3 and event/polling cases pass after it, covering normal short closes, TCP cooldown, URMA recovery and 64 KiB attachment echoes.
• C++14 compilation, the injection runner and fetched-SDK-header configuration checks passed.
• Prior URMA RM/RTP hardware runs passed TCP_ONLY, v2/v3 AUTO, mode switching, 64 KiB/1 MiB attachments and client SEND_POST injection across short/single/pooled and event/polling cases.

Related upstream work: send-buffer reclamation, polling lifetime and common URMA handshake.

Check List

• Build the library and run URMA regressions with mock CI.
• Add independent tests, preserve community UT sources and update documentation.
• Follow four-space indentation and C++ nullptr conventions.