Native lane: ownership rows for four unowned body-lowering fatal causes

[gunbai-bot]

In the broad native run at main bd2c586, 872 of 1058 file refusals carried a fatal cause with no FatalGrain row in v2.workflow.compile_door_cause_ownership known_frontier_causes, so gunbc.witness_v2_native_route native_route_file_refusals_attributed failed on them.

Rows added (FatalGrain, SharedSelfHostCriticalPath), each with its real owner:

• body_lowering_reason_call_argument_unread (802): value-position lowering. Function values in argument position go through MQ-1: function values in value position (lambdas and fn literals) lower to the generic fn Arrow; unwritten types are fresh type parameters #12210; the positional fold-step residue goes through MQ-1: a positional fold step lowers like the named form (position from std.algebra's fold signature) #12272.
• body_lowering_reason_operator_operand_unread (59): the body-lowering modeling pass (gentle-koi-724). The shapes are caret symbols and parenthesised as casts (v2: typed cast node (Transform[coerce, e] + <cast-target>) retires the as-cast refusal #12315).
• body_lowering_reason_type_annotation_not_carried (8): the let-annotation Bind carrier (v2: let-annotation Bind carrier (retires typed_statement_let rung drop) #12322) plus the typed cast node (v2: typed cast node (Transform[coerce, e] + <cast-target>) retires the as-cast refusal #12315).
• body_lowering_reason_paren_group_unread (3): raised by v2.compiler.body_lowering_fold body_lower_paren_group_lowered (introduced by XL-2 hotfix: call arguments lower as values; nested-call/record args no longer refuse the whole module (list args still refuse, located) #12173) when the group's inner value did not lower. The group has no semantics of its own, so this is value-position lowering and it flips with MQ-1: function values in value position (lambdas and fn literals) lower to the generic fn Arrow; unwritten types are fresh type parameters #12210 and v2: typed cast node (Transform[coerce, e] + <cast-target>) retires the as-cast refusal #12315.

Evidence (v2.test.v2_native_route, supplied receipt rows per the DESIGN §3 witness rule):

• the_body_lowering_frontier_causes_are_owned_at_fatal_grain: the file-refusal clause holds over one row per cause, and a Context-stage refusal classifies as CompilerFrontierAttributed.
• an_unknown_fatal_cause_beside_the_owned_ones_still_refuses_unowned: control. The same rows plus one unknown cause fail the clause.
• The existing an_unattributed_file_refusal_is_refused still pins the admission-level refusal. The admission check is unchanged.

Exclusions. classify_native_refusal looks exclusions up at FatalGrain, the same grain as file refusals, so no HeadGrain rows are needed. I matched the subject histogram neat-boar-16 sent from the bd2c586 receipt (rows checksum 9334399c71c35bc8). Before this PR, excluded_unattributed was 1388: Context call_argument_unread 1146, operator_operand_unread 206 and type_annotation_not_carried 19, plus 17 more. After it, 17 remain: infer_match_scrutinee_not_bool (2, Prepare: v2.test.extdeps.coordination_claims.coordination_obligation_exchange_arm_sharp_holds and ...coordination_obligation_settlement_arm_sharp_holds, the known residue) and eval_rejected_invalid_node (15, Eval). Neither has an evident frontier owner. eval_rejected_invalid_node is also overloaded: two claim-verdict arms in v2.compiler.eval relabel a Rejected actual as invalid_node, which erases the real cause. Whether to own them here is escalated to neat-boar-16 (my recommendation is follow-ups, not rows). value_unlowered carries its own row in #12299.

I could not run the claims locally (the sccache build failed), so CI's floor is the first execution.

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