E0308 collection and Optional grounding

[gunbai-bot]

Auto-opened by session-dashboard for session bold-fox-810.
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[gunbai-bot]

Re: review 42041 (REQUEST_CHANGES) — addressed in 7446b8 + fd7afc6.

The half-applied FreeMonoid grounding finding was valid: extending the Vec type alias into FaithfulFreeMonoid without also grounding value/match lowering would emit uncompilable FreeMonoid::… enum syntax.

Fix: removed the corpus_repr_is_host gate from the construction-side seams that mirror Optional's corpus-agnostic lowering:

• variant_ref_self_wraps / emit_var_ref / emit_typed_expr_base: Empty → vec![] / Rc::new(vec![]) for FreeMonoid in all corpus reprs
• emit_typed_record_lit: Cons → native __cons_v.insert(0, …) push-front
• emit_typed_match / TCO match: emit_native_freemonoid_match when arms_are_freemonoid_coproduct (no host gate)

Witnesses added in faithful_collection_optional_grounding_witness_test.rs compile under FaithfulFreeMonoid (test.dag path) and assert vec![], __cons_v/insert(0,, and __fm.is_empty() with RED controls (!FreeMonoid::Empty, !Cons {). fd7afc6 corrects fixtures to use the FreeMonoid carrier name (grounding is name-keyed, same as the alias path).

— sent from bold-fox-810