[M0] Transport abstraction and zenoh compatibility CI

[tetsuh]

Summary

Implement the Transport abstraction that hides the raw zenoh-cpp API from
the rest of the library, plus the dependency-upgrade CI that validates sitos
against both the minimum supported and the latest stable zenoh.

References

• docs/09_dependency_policy.md §3 (Transport API definition), §4.2 (upgrade CI)
• docs/02_architecture.md §1 (design rule: raw zenoh only inside src/transport/)
• docs/04_api_cpp.md §1 (Encoding, PutOptions, Subscription, Queryable)

Scope

• include/sitos/transport.hpp: Transport, TransportSample,
  TransportQuery exactly as specified in docs/09 §3
• src/transport/zenoh_transport.cpp: ZenohTransport implementation
  (put/delete/get/subscriber/queryable, Encoding schema, ack attachment)
• ZenohTransport::From(zenoh::Session&) factory for external session
  injection (requirement X04)
• Confirm the attachment API is usable with the locked zenoh version;
  record findings in the PR (fallback: ack-less put, per docs/09 §3)
• tests/integration/transport_test.cpp: round-trips through two
  transports in one process
• Static check in CI: no zenoh includes outside src/transport/
  (e.g. grep-based guard step)
• .github/workflows/dependency-upgrade.yml: nightly/manual matrix over
  {minimum supported, latest stable} zenoh

Out of scope

• StorageNode routing logic ([M2] StorageNode: queryable get/list skeleton #9, [M2] StorageNode: subscriber put/delete #10)
• ack protocol semantics ([M2] Ack protocol #14) — only the attachment plumbing here

Acceptance Criteria

1. transport_test is green with both the minimum supported and the latest
   stable zenoh versions
2. CI fails if any file outside src/transport/ includes raw zenoh headers
3. dependency-upgrade.yml runs on manual dispatch and reports per-version results

Dependencies

• [M0] zenoh-c/zenoh-cpp build integration #2

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/3-transport-adapter,
  Conventional Commits per §2.1, TDD: write transport_test RED first).
• This issue is closed automatically by the PR via Closes #3.