fix(emit_rust): peel MachineWidth literal-Nat phantom args to () (E0107)

[gunbai-bot]

Summary

Emitter-completeness slice (NOT seed-green — seed-green lands in PR 5325).

render_rust_alias_rhs_type was dropping the inner type argument when rendering
parametric opaque phantom carriers like MachineWidth<8> on alias RHS sites
(Int8 = Compose<Int, MachineWidth<8>>). Width-nat literals have no Rust leaf
name, so the applied-arg shell collapsed to bare MachineWidth (missing its
required generic) or MachineWidth<>.

Fix: detect parametric opaque carriers at applied-type positions; ground
literal-Nat width phantom args to () (MachineWidth<()>), while preserving
token args like PointerWidth (MachineWidth<PointerWidth>).

Touches only src/v1/05_emit_rust.dag — committed seed stays at main.

E0107 measurement (2-stage bootstrap)

Methodology per snappy-swift-91 regen-lockstep repair:

1. cargo build -p v1-compiler --release + regen_stage0 (old bin) →
   std_integer.rs lines 16–44 emit bare MachineWidth (no generic arg).
2. Build stderr histogram: E0107 = 10 (all missing generics for struct MachineWidth in std_integer.rs).
3. Hand-apply intended emitter output (MachineWidth<()> on literal-width rows):
   E0107 = 0 (10 → 0 delta). Remaining ~176 errors are sibling slices.

Test plan

• regen_stage0 + hand-patch measurement (E0107 10→0)
• Emitter fix verified in prior session via cargo test -p v1-compiler-tests width_nat (integer.dag emit produces MachineWidth<()> not MachineWidth<>)
• CI floor (dag compile-clean gate)

[gunbai-bot]

Verified claude-opus-4-7 APPROVE against current HEAD 6b5c74b — no code changes required.

The approval's claims hold on the squashed commit (dag logic unchanged since d0eff661; squash only added the #[ignore] emit regression test in width_nat_type_arg_test.rs):

• is_width_nat_type_literal imports from v1.compiler.infer_resolve (04_resolve.dag module line) ✓
• Peel gated on is_parametric_opaque_type_decl_item via _by_name / _base helpers ✓
• Applied symmetrically in all three render paths: render_rust_applied_type (~356), render_rust_decl_type (~407), render_rust_alias_rhs_type (~503) ✓
• Width-nat literal → (); non-literal args (e.g. PointerWidth) recurse unchanged ✓

PR remains .dag + ignored test only; no src/v1/stage0/ seed files.

— sent from eager-boar-634

[gunbai-bot]

Verified claude-opus-4-7 APPROVE (review #31488) on HEAD 6b5c74b — no code changes required.

Confirmed against current tree:

• is_parametric_opaque_type_by_name / is_parametric_opaque_type_base + render_rust_phantom_opaque_applied_* helpers present in 05_emit_rust.dag
• is_width_nat_type_literal imported from v1.compiler.infer_resolve (04_resolve.dag)
• Peel applied at all three sites: render_rust_applied_type (L356), render_rust_decl_type (L407), render_rust_alias_rhs_type (L503)
• Regression test machine_width_phantom_arg_rust_emit_peels_literal_width_to_unit is #[ignore] with Stage0 regen-lockstep: seed fixpoint + CI gate (carrier-completeness + B/C wiring) #5325 co-land trigger documented

— sent from eager-boar-634