fix: avoid iOS 17 filter sheet crash

[everpcpc]

Problem

Opening Filter & Sort on iOS 17 could crash inside SwiftUI AttributeGraph while it inspected Swift 6 function metadata for stored async loader closures in the metadata filter sheet. Newer systems did not reproduce the same runtime failure, which made the issue easy to miss outside iOS 17 validation.

Approach

Replace the reusable metadata filter sheet's stored async and transform closures with concrete source and display-style enums. The sheet still loads the same publishers, authors, genres, tags, and languages, but the SwiftUI view value no longer exposes async closure fields to AttributeGraph. The compatibility rule is also documented in AGENTS.md so future reusable SwiftUI views avoid this pattern.

Scope

• Add metadata filter source and display-style enums.
• Update MetadataFilterSection to use concrete source/display values instead of stored closures.
• Document the iOS 17 SwiftUI closure storage compatibility rule.

Validation

• make format
• make build-ios
• Launched on iOS 17.5 iPhone 13 mini simulator and opened Browse > Filter & Sort without a new crash report
• make build-macos
• make build-tvos
• git diff --check