emit: §3 single-authority for variant-import detection (dedup direct-vs-variant use, −15 E0252)

[briansrls]

What

Extracts the 4-rung variant-parent resolution (same-module decl → reexport surface → imported-enum scope → import-module-enum scope) into one helper import_variant_parent_for_name, used by both the variant-emission set (all_parents → parent_list) and the direct-import exclusion (non_variant_top).

Why (§3 single authority)

The direct-import filter used a weaker variant-detection authority (is_known_variant over type_summaries) than the variant-line emitter (find_variant_parent_in_module over typed_modules). They disagreed on cross-module / reexported variants, so a name like File (a UriScheme variant) was emitted both as a direct pub use crate::extdeps_uri::{File} and a variant use crate::extdeps_uri::UriScheme::{File} → E0252 reimport. One authority now decides, so a name can never appear in both forms.

Verified by execution (faithful v2+dsl census)

• E0252 444 → 429 (−15); the File/Https/Ftp direct-vs-variant collisions are gone (confirmed in emitted extdeps_external_authority.rs).
• Every other error class byte-identical (E0308=13239, E0277=982, E0432=27, …) — zero regressions.
• Full v1-compiler-tests suite (run-all gate parity, post-Widen the rust gate by construction (§1): invert the hand-picked 3-filter allowlist (29 of 792 green tests) → run-all-unless-#[ignore]d-with-written-reason; add CI-coverage-completeness so a new test is covered by default (fail-closed); measure CI-time impact before committing the full set #5427): 765 passed, 0 failed, 92 ignored.
• fmt clean; seed crate carries #![allow(clippy::all)].

Scope / out-of-scope

The remaining ~428 E0252 are a different root — emit_prelude() injects use crate::v1_rt::Witness into all modules while v2 std now defines Witness (E0255 redefinition + reimport cascade). That is the v1_rt-runtime-vs-std primitive-authority decision, routed to the value-grounding lane (stern-bat-782), not import hygiene. Region-local edit (import-emission ~2655); coordinated with the value-construct lane (~4693-5886).

Seed mirror (v1_compiler_emit_rust.rs) hand-synced to the .dag authority per current bootstrap protocol (regen --verify deferred, #5514).