Fix segfault when pattern matching on opaque types from lists

[lukewilliamboswell]

Summary

Fix segfault (issue #9113) when pattern matching on recursive opaque types extracted from lists. This PR addresses two related issues:

1. List element re-boxing (original fix)

When a recursive opaque type (like Element) is stored in a list, elements are auto-boxed for uniform size. When these elements are later used as tag union payloads, the code now correctly re-boxes them to match the expected layout.

2. Incorrect auto-boxing in record_collect (new fix)

The record_collect continuation was incorrectly auto-boxing fields based on whether the field's type is self-recursive. This caused layout mismatches when a separate recursive type (like NodeA) was used in a different type's record field (like NodeB's FoldB payload).

The fix removes this incorrect auto-boxing. The type-based layout is the source of truth:

• When a type references itself (Type := [Node({ child: Type })]), the layout system correctly creates Box(Type) during layout computation.
• When a separate recursive type is used ({ event: NodeA }), it should NOT be boxed because NodeA is already a complete type.

Changes

1. List re-boxing in tag_collect: When storing a list as a tag variant payload, if the expected element layout is box but the actual list has non-boxed elements, re-box each element
2. Remove incorrect auto-boxing in record_collect: Removed the logic that auto-boxed all self-recursive tag unions in record fields, which caused layout mismatches with the type-based layout
3. Dynamic discriminant offset: Use dynamic computation instead of stored value for recursive types

Test plan

• Added regression test test/fx/issue9113_simple.roc (tests list re-boxing)
• Added regression test test/fx/list_opaque_pattern_match_bug.roc (tests nested opaque types)
• All unit tests pass
• zig build minici passes

Fixes #9113

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