feat(ccu): integrate internal collective backend

[Kur0x]

Summary

• add runtime session, executor, and collective planner ownership
• integrate direct CCU as an internal collective backend
• preserve communicator fallback when direct CCU is unavailable
• add smoke infrastructure and backend/API boundary guards

Stack

5 of 9. Depends on codex/ccu-stack-04-runtime.

Validation

• git diff --check
• 75 focused backend, smoke-source, runner, and public-boundary tests passed; 3 skipped
• compiled and ran tests/comm/unit/test_tilexr_source_guards.cpp
• output: TileXR source guard checks passed

Scope

Host-only validation. Hardware smoke is not rerun in this PR.

[Chand1erLiu]

In production this condition is always true for a forced CCU request: none of the normal TileXRComm initialization paths call InitCcuBackend; the only caller is EnableCcuBackendForTest. Consequently the newly public TILEXR_COLLECTIVE_BACKEND_CCU option always returns TILEXR_ERROR_NOT_INITIALIZED. Please initialize the backend in the normal communicator lifecycle or lazily before dispatch.