emit import-closure: anonymous record-lit + variant payload harvest

[briansrls]

Summary

Extends reference_derived_use_lines import harvesting for the post-#7097 type-ref-import-missing class (#7153 follow-on). Adds five harvest paths in 05_emit_rust.dag (stage0 seed synced):

• anonymous_record_lit_surface_name — mirrors emitter find_struct_name_by_fields / find_unique_struct_name_by_fields for anonymous record lits (e.g. host_transport { primitive_type, bytes } → RuntimePrimitiveValue)
• collect_record_lit_field_struct_surfaces — variant field → payload struct types via variant_summary_key
• record_lit_variant_payload_struct_surfaces + expand_variant_payload_struct_imports
• Variant-aware collect_items_field_import_surface_names (RuntimeValue::RuntimePrimitive key)
• ExprRecordLit walk descends through field_init_node_value (not field-init wrappers)

Construction wall unchanged: candidates still gate on provider_proven_exports_symbol.

Probe receipt (post-change @ 0b0322a, CSSL_STD_SEED_LINK=1, six deep modules)

Module	1st error	E0433	E0422	E0425	E0109	E0107	E0391
03_resolve	E0422 RuntimePrimitiveValue	7	8	7	10	9	2
04_infer	E0422 RuntimePrimitiveValue	7	8	8	10	9	2
05_eval	E0422 RuntimePrimitiveValue	7	8	8	10	12	2
06_translate	E0422 RuntimePrimitiveValue	7	8	9	10	9	2
program_partition	E0422 RuntimePrimitiveValue	7	8	9	10	9	6
fold_lowering	E0422 RuntimePrimitiveValue	7	7	6	10	9	2

Lane totals: E0433=42 · E0422=47 · E0425=47 (unchanged vs post-#7153). 0/6 cargo-green. Histogram unchanged — harvest candidates not yet reaching synthesis for RuntimePrimitiveValue; follow-on checkpoint targets that gap.

Commands: ./target/release/regen_stage0 --verify (green) · parallel CSSL_STD_SEED_LINK=1 scripts/curated_cargo_probe_one.sh on all six modules.

Test plan

• regen_stage0 --verify — byte-identical, divergence_count=0
• Six-module curated cargo probe sweep (histogram above)
• CI floor (in progress on ready flip)