Add located failure receipts for regen_verify, compile_clean, and drift gates

[briansrls]

Summary

Floor gate failures (regen_verify_gate_passes, dag_compile_clean_gate_passes, generated_artifact_drift_gate_passes) previously surfaced only returned Bool(false) in CI — no file, path, or drift detail. This wires the existing _passes → _failure_receipt companion channel (same mechanism as extdeps_scope_placement_gate) for all three gates.

• regen_verify: captures regen_stage0 stderr (including Changed generated file(s): …) via WitnessBin.Run; records detail once, failure receipt consumes without re-running regen.
• compile_clean: extends the in-run compile receipt with the first hard diagnostic; consume_floor_compile_clean_gate_failure_detail projects it for the companion.
• generated_artifact_drift: records the path from the single observe_generated_artifact_population pass; companion reads retained detail instead of re-observing.

Seed-growth trade: This adds three host builtins (consume_regen_verify_gate_failure_detail, consume_generated_artifact_drift_gate_failure_detail, consume_floor_compile_clean_gate_failure_detail) plus interpreter registrations — surface growth on the Rust seed, which §7 says should shrink toward zero. The alternative is for each companion to re-derive failure by re-running the gate observation, which forks authority (§3) and, for regen_verify, would execute regen a second time — turning a diagnostic into expensive duplicate work. Retention is the cheaper and correct half; it extends an established channel rather than minting one: consume_floor_compile_clean_gate_verdict already exists on main with multiple call sites in cli_run.rs.

Carried main defect (not receipts work): Compiler-level touched paths widened this PR's discovery affected set enough to reach src/v2/test/lens_mock_totality/ for the first time, surfacing a pre-existing #7834 wall site on main. One-line fix in witness_template.dag: total_consumer_handles(case) — positional only on the function-value parameter. Diagnostic: call shape mismatch calling function value 'total_consumer_handles': named argument 'case' is not supported — use positional arguments. Censused: the only genuine site under src/v2/test/lens_mock_totality/. A second carried defect: dag/gunbc/plans/branch_merge_admission_model.dag was import-stripped and resolved only by pool-membership coincidence; whole-tree compile-clean exposes cell not found — restored explicit imports (same fix as #7857).

Receipt in production (this branch): Before: detail=returned Bool(false). After: detail=returned Bool(false) | compile-clean: dag/gunbc/plans/branch_merge_admission_model.dag:15:27: error: function 'cell' not found in scope — the located diagnostic the PR exists to produce.

Test plan

• cargo test -p v1-compiler floor_compile_clean_gate_refuses_on_failed_compile_receipt
• cargo test -p v1-compiler regen_verify_gate_failure_detail_* (round-trip + missing-record refusal)
• claim_batch --source-root dag --source-root src/v2 --entry dag/test/claim/floor_gate_failure_receipt_witness_test.dag — 7/7 PASS
• cron_mock_totality_test.dag resolves after witness_template positional fix

Positive controls (receipt fires with located detail): regen_verify_failure_detail_names_drifted_file_holds, generated_artifact_drift_verdict_names_planted_path_holds, dag_compile_clean_failure_reason failure arm in dag_compile_clean_failure_reason_empty_only_on_success, floor_gate_failure_receipt_companion_names_holds.

Negative controls (success path stays quiet — empty reason, companion not the story): regen_verify_failure_reason_empty_only_on_success, generated_artifact_drift_failure_reason_empty_only_on_success, dag_compile_clean_failure_reason_empty_only_on_success (each asserts the reason projection is "" on ExitSuccess only).

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