Fix RPC response validation and topic reallocation churn

[pavel-kirienko]

Summary

• Harden cy_respond() argument validation so malformed NULL-data/nonzero-size payloads and invalid breadcrumb priorities are rejected before platform unicast, without advancing the response sequence number.
• Preserve topic reader/writer handles when topic_allocate() changes eviction count but maps back to the same subject-ID, while leaving subject moves, displacement handling, gossip scheduling, and diagnostics semantics intact.
• Add intrusive regressions for both changes and move the L12/L17 TODO records into todo/resolved/.

Branch commits

• 62eea1d Fix cy_respond argument validation
• 0d29e96 Fix same-subject topic reallocation churn

Verification

• cmake --build build-l17 --parallel --target test_intrusive_topic_allocation_x64_c11 test_intrusive_topic_allocate_exhaustive_x64_c11
• ctest --test-dir build-l17 -R 'run_test_intrusive_topic_(allocation|allocate_exhaustive)_x64_c11' --output-on-failure
• cmake --build build-l17 --parallel
• ctest --test-dir build-l17 --output-on-failure (105/105 passed)
• git diff --check origin/main..dev
• C comment-line delta check: added_comment_lines=9 removed_comment_lines=11
• Review loop: 3 consecutive clean rounds, including Claude Fable 5/xhigh correctness review, no major/significant findings.