fix: check core files where the builder puts them

The pack integrity check rebuilt the core extras with no taken paths and
tested presence alone, so it credited core files at paths the platform's
own file holds and printed a core count the builder contradicted. It now
replays the builder's placement through packextras.hidden_extras and
compares each placed member with the copy the builder resolves.

wiki/testing-guide.md

@@ -38,7 +38,7 @@ Every module in `tests/`, by the area it guards.
 | Core regression suites | synthetic | `test_e2e`, `test_audit_regressions`, `test_tool_messages` |
 | File resolution and identity | synthetic + collection | `test_adler32_resolution`, `test_agnostic_scan`, `test_agnostic_scan_mode`, `test_agnostic_scope`, `test_agnostic_substitute`, `test_archive_choice`, `test_bios_companions`, `test_bundled_identity`, `test_casefold_cache`, `test_db_aliases`, `test_declared_names`, `test_derivations`, `test_directory_entries`, `test_extra_identity`, `test_foreign_tree`, `test_foreign_tree_names`, `test_frontend_digest`, `test_game_data_homonyms`, `test_hashing`, `test_identity`, `test_md5_pack_context`, `test_name_tiebreak`, `test_nec_filing`, `test_owner_resolution`, `test_path_suffix`, `test_resolution_evidence`, `test_shipped_content`, `test_twin_declarations`, `test_validated_choice`, `test_variant_choice`, `test_variant_identity`, `test_zipped_member` |
 | Verification, cross-reference and profile checks | synthetic + profiles | `test_crypto_aes`, `test_crypto_unique_data`, `test_crypto_verify`, `test_case_insensitive_platforms`, `test_checks_have_values`, `test_core_file_fills_declaration`, `test_cross_reference_sources`, `test_file_consumers`, `test_fileless_audit`, `test_gap_settlement`, `test_hidden_extras`, `test_launcher_layer`, `test_native_mode`, `test_no_case_collisions`, `test_path_conflict_status`, `test_path_null_copies`, `test_profile_checks`, `test_romm_hash_rule`, `test_shared_groups`, `test_standalone_mode`, `test_undeclared_dedup`, `test_unplaced_extras`, `test_validation_scope`, `test_verify_all_dir`, `test_verify_emulator_evidence`, `test_verify_listing`, `test_verify_pack_choice` |
-| Packs and manifests | synthetic + real packs | `test_artifact_lock`, `test_atomic_artifacts`, `test_batocera_cores`, `test_deterministic_zip`, `test_emulator_pack_counts`, `test_finalize_pack_names`, `test_large_file_cache`, `test_libretro_layout`, `test_manifest_entry`, `test_pack_conformance_dirs`, `test_pack_counts`, `test_pack_destinations`, `test_pack_failures`, `test_pack_file_names`, `test_pack_grouping`, `test_pack_integrity`, `test_pack_member_identity`, `test_pack_readme`, `test_platform_inheritance`, `test_platform_keys`, `test_required_only_archives`, `test_required_only_twins`, `test_split_extras`, `test_split_names`, `test_split_pack`, `test_system_pack_extras`, `test_system_pack_parity`, `test_torrentzip`, `test_which_pack` |
+| Packs and manifests | synthetic + real packs | `test_artifact_lock`, `test_atomic_artifacts`, `test_batocera_cores`, `test_deterministic_zip`, `test_emulator_pack_counts`, `test_finalize_pack_names`, `test_large_file_cache`, `test_libretro_layout`, `test_manifest_entry`, `test_pack_conformance_dirs`, `test_pack_core_extras`, `test_pack_counts`, `test_pack_destinations`, `test_pack_failures`, `test_pack_file_names`, `test_pack_grouping`, `test_pack_integrity`, `test_pack_member_identity`, `test_pack_readme`, `test_platform_inheritance`, `test_platform_keys`, `test_required_only_archives`, `test_required_only_twins`, `test_split_extras`, `test_split_names`, `test_split_pack`, `test_system_pack_extras`, `test_system_pack_parity`, `test_torrentzip`, `test_which_pack` |
 | Slots and regions | synthetic + profiles | `test_region`, `test_region_agnostic_scan`, `test_region_groups`, `test_region_held`, `test_region_platform_index`, `test_region_required_only`, `test_slot`, `test_slots`, `test_slots_shipped_file` |
 | Hardware targets and systems | synthetic + target files | `test_extract_targets`, `test_lakka_targets`, `test_libretro_slugs`, `test_system_ids`, `test_system_selection`, `test_target_names`, `test_target_scrapers`, `test_target_systems`, `test_target_verify` |
 | Installer and downloads | loopback server | `test_check_release`, `test_download`, `test_install`, `test_install_cores`, `test_install_experience`, `test_install_selection` |