feat(ccu): add two-rank direct alltoall

[Kur0x]

Summary

• add two-rank direct CCU AllToAll program generation
• add endpoint exchange, remote-buffer import, and route-scoped memory overrides
• add synchronization fixes and repeated submission support

Stack

7 of 9. Depends on codex/ccu-stack-06-signal-wait.

Validation

• git diff --check
• focused AllToAll, backend, orchestrator, smoke, driver, and lower-layer tests
• the suite exercised 169 tests; 168 passed before one outdated source assertion was aligned with the final route index
• the corrected assertion was rerun and passed

Scope

Prior two-rank hardware evidence belongs to #71 and was not rerun from this host.

[Chand1erLiu]

This fixed capacity only covers 64 copy blocks (64 * 32 KiB = 2 MiB), while the request validation accepts any aligned nonzero bytes. For a larger all-to-all, the generated program exceeds the allocated repository/task window and fails later despite passing validation. Please derive the instruction capacity from bytes with overflow and uint16_t window checks, or reject sizes above the supported limit up front.