refactor(rtps): the participant lives below both callers that publish through it

[cagataycali]

RtpsRobot opened a DDS writer by importing the use_rtps @tool - the last drivers|mesh -> tools inversion that was a transport rather than a verb package. The participant (one DomainParticipant, writers/readers cached per (topic, type), the IDL sample builder, the action dispatch) moves to strands_robots/rtps/participant.py, beside the mangling and IDL bundle it is built on. The tool keeps what an agent envelope owns: the numeric-option domains, the operator gate, the docstring a model reads.

before                                   after
mesh.rtps_robot -> tools.use_rtps        mesh.rtps_robot -> rtps.participant
                  (drivers|mesh -> tools)              (same layer)

measure	before	after
upward runtime edges	4	3
drivers|mesh -> tools	4	3
runtime cycles	0	0
tools/use_rtps.py	329	138
package LOC	202,471	202,610
test LOC	553,659	553,670

+139 package lines are the new module's own contract: why it sits there, the GATE_TOOL label, and rtps_action's required gate argument. Serves #3818 0.8-0.9 (layered DAG, zero cycles as a test).

Safety is unchanged and now has one owner. rtps_action takes the operator gate as a required keyword argument - a caller cannot publish to a blocklisted surface by forgetting one - and both callers pass the same GATE_TOOL label, so one cmd_vel incident files one interrupt id and one audit source whichever surface asked.

Tests. Pre-fix control (new test files on a clean main worktree): 3 failed / 19 passed, and the cross-transport gate suite cannot import the participant at all; after: 22 / 33 passed. New pins - a rtps.participant row in the layer-contract table (placement, nothing read above its layer, callers in two layers), the required-gate signature, and a robot publish prompting byte-identically to the tool's. scripts/check_whole_tree_graders.py 6196 passed / 77 skipped; tests/mesh tests/tools tests/rtps + the root graders 10048 passed / 24 skipped; ruff + mypy clean on 2,205 files.

[yinsong1986]

Summary

Moves the DDS mechanics (shared DomainParticipant, per-(topic, type) writer/reader caches, IDL sample builder, action dispatch) out of the use_rtps @tool into strands_robots/rtps/participant.py, so RtpsRobot publishes through the participant directly instead of importing an agent tool from the layer above — eliminating the last drivers|mesh -> tools transport inversion. The safety contract is preserved and strengthened: rtps_action takes the operator gate as a required keyword-only argument (no caller can silently omit it), both callers key it with the same GATE_TOOL label so one blocklisted topic files one interrupt id and one audit row, the gate is consulted after the backend probe and before the process-wide lock, and topic/type validation via ros_topic_error/dds_type_name now sits in the participant where both callers hit it. The moved code is byte-equivalent to what lived in the tool; behavior visible to callers is unchanged.

What's good

• The required-gate signature plus the never_gated singleton makes the un-gated-caller failure mode unrepresentable, and tests/test_every_ros_transport_consults_the_command_gate.py pins both the signature (keyword-only, no default) and byte-identical operator prompts across the tool and robot surfaces.
• The layer contract table in tests/test_import_layers_are_a_dag.py gains a rtps.participant row with an equality assertion, so moving the participant back up into tools fails a test rather than silently restoring the inversion.
• The obsolete mesh.rtps_robot -> tools.use_rtps exception is removed from scripts/check_import_layers.py rather than left as a vestige.
• Test seams were migrated consistently (_backend, time.sleep, rtps_action patch points all repointed to the participant module) and the pre-fix control run documented in the description shows the new pins fail on main.