fix(view): preserve packed closed-enum unknowns

[ogarciarevett]

Closed proto2 enums already preserve unknown values on the owned path, but the packed repeated view path was still dropping them.

This keeps those packed unknown values by letting UnknownFieldsView hold synthetic unknown fields for cases where there is no borrowable per-element wire span.

I also added a regression test for the packed repeated view case and removed the corresponding item from the README limitations list.

Proof:

• RED on main in a clean worktree: cargo test -p buffa-test 'tests::closed_enum::test_view_closed_enum_repeated_packed_unknown_preserved' -- --exact
• GREEN here: cargo test -p buffa-test 'tests::closed_enum::test_view_closed_enum_repeated_packed_unknown_preserved' -- --exact
• Runtime coverage: cargo test -p buffa 'view::tests::unknown_fields_view_to_owned_includes_owned_fields' -- --exact
• Broader validation: cargo clippy --workspace --all-targets -- -D warnings and cargo test --workspace

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