Fixes #329.

This fixes #[derive(BorshSchema)] for enum variants that contain PhantomData<T>.

What changed:

• added a schema-only mode for generic parameter detection that can include PhantomData<T> when enum variant schema params are computed
• kept the existing PhantomData<T> skip behavior for the other derive paths
• added a regression test for enum Parametrized<T> { Item(PhantomData<T>) } using a marker type that does not implement BorshSchema

Why:
The struct case already works, but the enum variant case from #329 expands into code that tries to use the outer generic parameter from a generated inner item. I checked this with a throwaway crate: clean origin/master fails with E0401, while the patched local Borsh passes and can call MyEnum::<NotSchema>::add_definitions_recursively.

Checked:

• cargo test -p borsh --features unstable__schema schema::test_phantom_data
• cargo test -p borsh --features unstable__schema schema::test_generic_enums
• cargo test -p borsh --features unstable__schema
• cargo test -p borsh-derive --features schema
• cargo fmt --check
• cargo clippy -p borsh-derive --features schema -- -D warnings
• cargo clippy -p borsh --features unstable__schema -- -D warnings