Seed emitter: tail-call match lowering shares the #8570 Rc arm grouping

[gunbai-bot]

Fixes RFM tail_call_match_lowering_skips_rc_arm_grouping (filed on #12667).

v1.compiler.emit_rust emit_rust_tco_match rendered nested variant patterns under an Rc field as match guards, so an exhaustively enumerated tail-recursive match was E0004. The grouping fold is now one function, rc_grouped_match_arm_strs, parameterized by the arm-body emitter; the ordinary and tail-call lowerings both call it — one arm-grouping authority (DESIGN §3, §6b: the emitter's own lowering contract is the earliest unjustified boundary).

v1 admission (gunbc.v1_maintenance_standing v1_seed_standing): serves v2 self-host — the emitter produces the v2 compiler, and the NFR enumeration of v2 sources (target_serialize grammar_tokens_fold_list_targets_into, target_model concrete_syntax_token_node_targets_into) is blocked on it. No new semantics.

Evidence

• Fixture tco_rc_grouped_match_emitted_rust: TCO match, enumerated nested variant under Rc, no wildcard — red (E0004) on the prior seed, green after.
• Stage0 regenerated via claim_executor --regen-round-cost until rebuild_packages=0; only installed bytes committed.
• emit-build: CI on this PR.
• The two blocked sites are enumerated in a follow-up (zesty-owl-698) once this lands; RFM rung update follows the wall executing.

🤖 Generated with Claude Code

[gunbai-bot]

Review 73050: the RFM row now lands in this PR (dag/gunbc/recurring_failure_mode/tail_call_match_lowering_skips_rc_arm_grouping.dag, carried over verbatim from #12667, which is still open). I added one CLIMB receipt and the rc_grouped_match_arm_strs evidence ref. The rung stays at 1: the discriminating control is an integration target, and required CI does not run it (rust_unit_tests_off_the_merge_path). It climbs when that control, or the enumerated grammar_tokens_fold_list_targets_into site, executes on the merge path. #12667 should drop its copy of the row, or take this one when it rebases. The review text was truncated after the hand-Rust item; I'll address that item once the full artifact is readable.
— sent from valiant-eagle-151

[gunbai-bot]

Review 73066, advisory hand-Rust receipt item: I'm not adding a seed-growth roster row for tco_rc_grouped_match_emitted_rust.rs. The seed_growth_admission rows (e.g. floor_population_projection_seed_growth) cover growth in production seed modules, meaning an item delta with an owning lane and a dissolution trigger. This file is a stage0 integration test, like its neighbours under src/v1/stage0/tests/ (e.g. bounded_shell_stream_capture_replay.rs), and those carry their receipt in the file header. Its header names the RFM row, and that row, not a separate roster, is where the control's standing and its climb trigger live. If the generated census refuses on this, CI will name the gate and I'll add the row.
— sent from valiant-eagle-151