Census: runner_browser_toolchain_converge_ci_wet does not select (unblocks every fleet plan)

[gunbai-bot]

Every fleet-converge plan on main refuses with CanonicalProtocolRefused — gunbc.host_convergence_protocol bind_canonical_protocol answered CensusProtocolUnbound. Examples are the srv1 plans in runs 36562381449 and 36563688759 at e0d0e01efb. This blocks every fleet deploy, including taking #12482 live.

Cause. #12500 added a gunbc.host_convergence_census row for gunbc.runner_browser_toolchain runner_browser_toolchain_converge_ci_wet with protocol_phases(true, true, …), which declares both selects and observes. admit_protocol_phases refuses that combination as SelectionLeakedIntoObservation, so bind_census_protocol_phases answers CensusSelectionLeaked for that row, and fleet_converge_plan_cli refuses to plan. A probe on main confirmed that exact row.

Fix. Set selects to false. The mode chooses no realization:

• It admits a host into the pool derived from the floor job's runs-on labels (runner_browser_toolchain_host_admission); admits was already declared.
• It observes superseded trees, reads back, and checks readiness.

Selection belongs to std.decision's row alone, which is the only other row declaring selects. The rest of #12500 is untouched.

Evidence, executed locally on main plus this fix:

• The probe reports bind_census_protocol_phases → CensusPhasesBound and canonical_protocol_gate → CanonicalProtocolAdmitted. On main without the fix it reported CensusSelectionLeaked at this row.
• test.claim.host_convergence_protocol_witness canonical_protocol_binds_with_apply_awaiting and production_gate_admits_the_canonical_protocol both return true.

witty-crane-254 (#12500's owner) is asked to review.

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