required_floor.dag: DeclinedNoCiWetLane arm with census control

[gunbai-bot]

Adds DeclinedNoCiWetLane { pattern } to the .dag type v2.workflow.required_floor RequiredFloorDisposition, mirroring the Rust arm landed by #12794/#12833.

• required_floor_disposition_with_changed_selection takes declared_wet_pattern: Optional<String>. A selected row with Present becomes DeclinedNoCiWetLane.
• v2.workflow.floor_changed_witness gains changed_witness_decline_blocks, an exhaustive match that returns false only for this arm. standing_blocks consumes it.
• Census control: gunbc.discovery_census CensusCounts.declined_no_ci_wet_lane is its own bucket, and its row partitions into the declined side. The arm is also carried through FloorStandingCounts, v1_witness_census, and the exhaustiveness fixtures.
• New tests:
  • a_selected_declared_bin_wet_row_is_declined_no_ci_wet_lane
  • the_no_ci_wet_lane_decline_does_not_block_and_other_declines_do
  • the_census_counts_a_declined_no_ci_wet_lane_row_in_its_own_bucket (uses a real hermetic-site row)

Evidence (gunbc run --claim-run, per file):

File	Branch	Main
floor_changed_witness	51/51	—
gate_selector	4/4	—
exhaustiveness	26/26	—
discovery_census_witness	13 pass / 10 fail	12 pass / 10 fail (same 10, at c95f906)
effect_demand_floor_join	9 pass / 3 fail	9 pass / 3 fail (same 3)

The failures in the last two rows are pre-existing and not caused by this change.

Gaps found, not fixed here:

• The .dag type still lacks the Rust arm DeclinedChangedWitnessOutsideDiscovery.
• No .dag mirror exists for the Rust join predicate decides_a_changed_selection.

Reviewer: deep-ferret-305.

Narrowing the countable (deep-ferret-305, option C). Four rung drops name w_RED_the_deleted_lanes_do_not_return as their countable. It used to live in test.claim.witness_floor_workflow_consolidation_witness_test, whose import closure is the whole workflow emitter. The 2026-09-19 nominal roster withdrew that module, so the countable stopped running on any required path.

• Lane facts. The facts the countable reads moved to a new module, gunbc.witness_floor_lanes: job ids, lane variables, required_lanes_roster, witness_floor_lane_roster, witness_floor_lane_job_ids and RequiredLane. gunbc.witness_floor_workflow and gunbc.required_lanes_gate import them back, so the emitted workflow bytes are unchanged; the generated lane checks that.
• The claim. It moved to test.claim.deleted_lanes_countable_witness_test, which the gate admits at exact grain.
• Citations. The four rung drops, the census fixture, the grandfathered roster and the gate comments now cite the new identity. docs/design-rung-drops.md is regenerated.

Measured gate delta. I assembled the floor's nominal prepared subject on the real pool with required_floor_nominal_subject_seeds and assemble_prepared_subject_closure. This is the subject a run prepares when it touches nothing, so no diff seeds are involved:

Admitted module	Nominal subject (modules)	Added
Neither	2,281	—
deleted_lanes_countable_witness_test	2,282	+1 (the countable module itself)
Consolidation module instead	2,362	+81

The figures come from a one-off test run on c80e644 and are not committed.

Fixture ownership. The stale census and floor-join fixtures belong to #12857. That PR lands first, and this one then merges main.

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