[M0] zenoh-c/zenoh-cpp build integration

[tetsuh]

Summary

Integrate zenoh-c / zenoh-cpp into the CMake build with a pinned (locked)
version, and prove the toolchain works with a minimal smoke test.

References

• docs/06_build_test_packaging.md §2 (dependency resolution)
• docs/09_dependency_policy.md §2 (supported version range), §4.1 (locked CI version)

Scope

• Fetch zenoh-c prebuilt releases via find_package(zenohc); FetchContent
  fallback with the version pinned in one place (e.g. cmake/zenoh_version.cmake)
• zenoh-cpp header integration
• Corrosion (Rust source build) path documented as an alternative for
  environments without prebuilt binaries (documentation only, not CI)
• Smoke test: open and close a zenoh session (tests/integration/zenoh_smoke_test.cpp)
• CI runs the smoke test on both Windows (MSVC) and Linux (gcc, clang)

Out of scope

• Transport abstraction ([M0] Transport abstraction and zenoh compatibility CI #3)
• Any sitos library code using zenoh

Acceptance Criteria

1. CI is green on both OS with the zenoh session open/close smoke test
2. The pinned zenoh version is defined in exactly one place
3. Build works from a clean checkout with no preinstalled zenoh

Dependencies

• [M0] Repository bootstrap and CI skeleton #1

For the implementer

• Copy the checklist above into your PR description and tick items as you
  complete them. Do not edit the checklist in this issue.
• Follow docs/development_workflow.md (branch issue/2-zenoh-build,
  Conventional Commits per §2.1, TDD where applicable).
• This issue is closed automatically by the PR via Closes #2.